From MozillaZine Knowledge Base
Adobe Flash (formerly Macromedia Flash), refers to the Flash Player and browser plugin needed to handle SWF videos, web page animations and interactive content. Even though the Flash plugin is also referred to as "Shockwave Flash", it should not be confused with the Shockwave plugin that handles "Shockwave for Director" content.
Enabling and disabling Flash
Starting in Firefox 3 and in SeaMonkey 2, installed plugins, including Flash, can be disabled and enabled in the Add-ons manager. To disable or re-enable Flash, go to "Tools -> Add-ons (Add-on Manager) -> Plugins" and find the "Shockwave Flash" entry, shown here when enabled. Click the "Disable" button to disable the Flash plugin. If Flash is disabled, click the "Enable" button to re-enable Flash.
Note: If the Flash plugin is installed, make sure it is enabled in the Firefox (3.0 and above) or SeaMonkey 2 Add-ons Manager (see above); otherwise, the Flash test will not work and the "Shockwave Flash" plugin entry will not be displayed in the about:plugins list.
To see if the Flash plugin is working and what version is being used, visit one of these pages:
If Flash is not installed, this may trigger the Plug-in Finder Service, which you can use to download and install the Flash plugin on Windows (see below). At time of writing, the Plug-in Finder Service is triggered but fails to install the Flash plugin on Linux or Mac OS in Firefox 3 and above (you will need to manually download and install the plugin).  
You can also see what version (if any) of the Flash plugin is being detected using about:plugins (see the linked article for details). If Flash is installed and enabled, the about:plugins list will include an entry similar to:
In Flash versions 11.2 and above, the file name will include the plugin version; for example, for Flash plugin version 126.96.36.199 on Windows, the plugin file name would be "NPSWF32_11_2_202_235.dll" (instead of "NPSWF32.dll") .
If you want to see the full path to the plugin file, as shown in the above Windows example, change the value of plugin.expose_full_path to true using about:config . Caution: This setting may have security implications so you should reset the preference to "false" when you are done. 
Note: The about:plugins list is generated from data stored in the "pluginreg.dat" file. In some cases, the stored data may be incorrect.  To refresh the list, close the browser and delete the "pluginreg.dat" file, located in the profile folder in current Mozilla browsers.
Installing or updating Flash
To install Flash or to update to the latest version for your operating system, visit the Adobe Flash Player Download Center (see the system requirements and FAQ pages for more information). Adobe has recently started to bundle other applications from other vendors with the download so make sure to opt out of any programs or tools you don't want before clicking the button. If you cannot find a Flash Player download for your OS, find it manually on this page or here.
After downloading the Flash installer, completely close your Mozilla application and run the installer. The recent installers are only helper applications downloading the actual application from the Adobe site, thus they are smaller in size than previous downloads. In current versions, the INSTALL button is disabled until you check-mark the box in the Adobe Flash Player installation dialog where it says, "I have read and agree to the terms of the license agreement". See this blogspot article for details, including a screenshot.
Starting with Flash Player 11.2 on Windows, an option for silent, automatic updates is selected by default when you install Flash. This feature will download and install Flash Player updates in the background, without any user interaction. See this Adobe article for more information. 
Note: If you want to download the latest Flash 10 installer, see this Abobe forum post. Also note that, for Mac OS, Flash 10.2 and above is Intel-only. Flash 10.1 r102 is the last supported version for PPC Mac.  Flash Player 9 is no longer supported. Archived Flash versions can be downloaded here.
Installing Flash on Windows
If the Flash plugin is not installed and you visit a page that has Flash content that requires the plugin, the Plug-in Finder Service can download and install Flash for you (see Installing the Flash plugin (Firefox Support) for more information). If you want to try this method and Flash is already installed, you can uninstall Flash and then visit Adobe's Flash test page to launch the Plugin Finder Service ; however, it may offer you an outdated version.
You can also install Flash or update to the latest version for your operating system by downloading the installer from the Adobe's Flash Player download page. Alternately, you can use this link for the latest Flash 11 plugin installer. If you need Flash 10, you can use this link for the latest Flash 10 plugin installer (both downloads are linked from the Adobe Flash Player distribution page).
Once the download completes, exit Firefox and run the installer. You normally don't need to uninstall Flash before updating to the latest version, since the Flash installer automatically removes the older version. However, you will need to uninstall your current Flash version if you are downgrading from a higher version (e.g., from Flash 11.2 to Flash 10.3).
Note: There are two Flash players offered for Windows users: the Flash ActiveX control for Internet Explorer/AOL and the Flash plugin for Firefox/Mozilla/Opera browsers. Each must be installed separately, since the Flash plugin installer does not add the ActiveX control and vice versa. In other words, it's possible to have the Flash 10 plugin installed for Mozilla browsers and the Flash 11 ActiveX control installed for Internet Explorer, and both will work.
Flash 9.0.r28 and earlier versions of the Flash plugin installer copied these two files to the installation directory of Firefox and other detected Mozilla browsers:
You would then have needed to copy these two files to the plugins folder of undetected browsers (e.g., zip builds). Note that an XPI package that was available for Flash 9.0.r28 and earlier using the "Install" link at PluginDoc also copied the Flash plugin files to the Windows System directory's Macromed\Flash folder and added a registry entry so that all Mozilla browsers could find the files through plugin scanning, using a Windows registry PLID scan 
Starting with Flash 9.0.r45 the Flash plugin installer for Firefox and other Mozilla-based browsers does not copy any files to the browser plugins folder. Instead, the files "NPSWF32.dll", "flashplayer.xpt" and related files are placed in the Macromed\Flash folder located in the Windows System directory (e.g., C:\WINDOWS\SYSTEM32\Macromed\Flash) and the installer removes any copies found in the installation directory of detected browsers. A Windows registry entry is created enabling each Mozilla browser to detect Flash via plugin scanning (PLID scan). 
Windows installation issues
For other installation issues, see the Adobe article Installation problems | Flash Player | Windows.
An uninstaller can be downloaded from Adobe.com to remove Flash from Mac or Windows systems. After downloading, make sure you completely exit your Mozilla browser and close all other Internet applications before you run the uninstaller.
See Flash Player Help | Uninstall Flash Player | Mac at Adobe.com for a link to download the uninstaller and detailed instructions.
See Flash Player Help | Uninstall Flash Player | Windows at Adobe.com for a link to download the uninstaller and detailed instructions. Adobe's uninstaller will remove Flash from your system, including both the Flash Player plugin for Mozilla applications and the Flash Player ActiveX control used by Internet Explorer and AOL.
Warning: Some software may not work properly unless the Flash ActiveX control is installed.   If you only want to remove the Flash plugin for Firefox and other Mozilla applications, try removing the "Adobe Flash Player Plugin" from the Windows Control Panel list of installed programs (see below).
Uninstall using Windows Control Panel
Flash 9.0 r45 and above: You can uninstall the Flash plugin for Mozilla applications (and keep the Flash ActiveX control, if installed) by selecting and removing just the "Adobe Flash Player Plugin" entry from the Windows Control Panel list of installed programs (e.g., Windows XP "Add or Remove Programs" or Windows 7/Vista "Programs and Features" list).  Note: If you see an entry for "Adobe Flash Player ActiveX", do not select it, as this is for the Flash ActiveX control needed for Internet Explorer and AOL.
Flash 9.0 r28 and below: Your Windows list of installed programs may have an entry for "Adobe Flash Player ActiveX" but this will only remove Flash for Internet Explorer and AOL. To completely remove Flash from your system, download the Flash uninstaller from Adobe.com. To remove Flash from Mozilla browsers only, delete these Flash plugin files from all locations. 
Linux and Solaris
If you installed via Adobe installation routine, follow the uninstall instructions in the Flash Player ReadMe for your system, linked below:
If you installed via apt-get/aptitude (Debian/Ubuntu): sudo apt-get remove flashplugin-nonfree
If the uninstall don't work: 1. type about:config in the address bar and press Enter. Find the option plugin.expose_full_path and change the value to "true" (double-clicking the preference name will toggle the setting). 2. type about:plugins and locate the flash plugin. Remove the plugin files (both .so and .xpt)
Flash Settings Manager
You can adjust your Flash settings in the Flash Player Settings Manager. Some settings can be accessed from the context (right-click) menu of a Flash object during playback. Other settings, such as global settings for privacy, storage, security, and automatic notification of Flash Player updates, are accessible via links in the Adobe support document, Flash Player Help - Settings Manager
If Flash isn't working properly, first make sure that the latest version of Flash is installed and enabled (see above) before trying the following solutions. If your problem isn't listed below:
Warning: Installing an older version of Flash may be suggested as a workaround for some Flash issues; however, this can make your system vulnerable to security exploits that have been addressed in the latest Flash version. 
Flash Player 11.3 Protected Mode - Windows
Starting in Flash Player 11.3, the Flash plugin used in current versions of Firefox and SeaMonkey runs in a Protected Mode in Windows Vista and above. Note that, when Flash is running in Protected Mode, you will see two "FlashPlayerPlugin_<version>.exe processes in Windows Task Manager (e.g., “FlashPlayerPlugin_11_300_257.exe”). This new security feature can cause crashes, freezes, An error occurred. Please try again later messages at YouTube, or other problems with Flash content. A recent Mozilla update has resolved a crash at exit problem (see bug 747683, fixed in Firefox 13.0.1 and SeaMonkey 2.10.1). Mozilla and Adobe are working on other issues, so make sure to keep Firefox, SeaMonkey, and Adobe Flash updated to take advantage of bug fixes.   
See Adobe's How do I troubleshoot Flash Player's protected mode for Firefox? Flash Player FAQs page and this Flash Player Help article for suggested diagnostic steps and workarounds. Note that the RealPlayer Browser Record Plugin extension was added to the Add-ons Blocklist on June 12, 2012 and June 14, 2012 and should be disabled in the Add-ons Manager. Even so, you may also need to open RealPlayer, go to "Preferences -> Download & Recording" and uncheck "Enable Web Download & Recording for these installed browsers", then click OK, close RealPlayer and restart Firefox or SeaMonkey (see bugs 763459#c19 and 764210#c24). If you don't have RealPlayer installed or if unchecking Web Download & Recording in RealPlayer preferences doesn't help, you should troubleshoot other extension issues using the Safe Mode article instructions to restart in Firefox or SeaMonkey Safe Mode; then click "Continue in Safe Mode". If Flash works without problems, check for problematic extensions (see Extension issues below).
As a last resort, you can either temporarily disable Protected Mode in Flash 11.3 (see below) or you can uninstall Flash and then install a previous version. See Adobe's How do I revert to a previous version of Flash Player? page for details. Note that the latest Flash 10.3 version (available here) includes all known security patches. If you wish to install a Flash 11.2 version, even though it is less secure, see this forum post for Flash 188.8.131.52 download links.
Disabling Protected Mode in Flash 11.3
Instead of using the other workarounds suggested above, you can temporarily disable Protected Mode in Flash 11.3 by adding the line ProtectedMode=0 to the Flash "mms.cfg" file located in:
For example, on a Windows 7 64-bit system, open the C:\Windows\SysWOW64\Macromed\Flash folder, copy the file "mms.cfg" to the desktop, open it in Notepad, add a new line ProtectedMode=0 and then close Notepad, saving your changes. Next, go back to the C:\Windows\SysWOW64\Macromed\Flash folder, rename the original file to "mms.cfgBAK" (to save it as a backup) and then drag and drop in the modified "mms.cfg" file from the desktop. You can later re-enable protected mode by either restoring the backup or by editing the mms.cfg file to remove the ProtectedMode=0 line you added.     
Note: The Flash 11.3.300.262 plugin installer from this direct link apparently disables Protected Mode by default and requires adding ProtectedMode=1 to the Flash "mms.cfg" file to enable it. See this support.mozilla.org discussion thread for more information.
The plugin for this content has been disabled
If you see a box with the message, The plugin for this content has been disabled. Click here to manage your plugins instead of Flash content on web pages, make sure that "Shockwave Flash" has not been disabled in the Add-ons manager Plugins list (SeaMonkey 2 or Firefox 3 and above). Also check in about:plugins and make sure that Shockwave Flash is properly detected; if it isn't, close the browser and delete the file "pluginreg.dat" from the profile folder. 
Flash player not working on certain websites
Note: For issues specific to YouTube Flash videos, visit the YouTube Help Center.
If Flash video or other Flash content plays on some websites but not others, the website may be experiencing heavy traffic or having other problems. Try clearing the cache and reloading the video and then try the suggestions listed below (read this article for other solutions).
Check Flash storage settings
Some websites may require that Local Shared Objects (Flash cookies) are enabled in your Flash settings. To check the Flash local storage settings for a specific site, right-click the Flash content that does not play and select "Settings", then click the Local Storage icon. Also open the Global Settings Manager to ensure that you have specified disk space to allow third-party content to store data on your computer.  Note: Starting in Firefox 4, if Flash Player 10.3 or above is installed you can clear Flash cookies from within Firefox, via (Tools) "Clear Recent History -> Everything -> Cookies" (bug 625496).
Check user agent
Flash content may also fail to load on certain websites if you use a "spoofed" user agent (UA). For example, if you go to the Firefox menu and click "Help -> About Mozilla Firefox" and it shows
Flash issues in Firefox 3.6.4 and above
Starting in Firefox 3.6.4 on Windows and Linux, the Flash plugin is isolated from the browser process and runs in a separate "plugin-container" process ("plugin-container.exe" on Windows). See Plugin-container and out-of-process plugins for more information about this new crash protection feature and how to disable it to troubleshoot Flash issues.
Flash content blocked - Firefox 3.6.4 and above
The Adobe Flash plugin has crashed - Firefox 3.6.4 and above
If you see a dialog box with the message, The Adobe Flash plugin has crashed instead of Flash content, either the plugin has crashed or Firefox has detected that it has stopped responding. Try the suggestion here and in this article to turn off the hang detector. If you still have problems, try turning off hardware acceleration in your Flash settings (see below). 
Flash does not work in Firefox 4 - Mac OS
Starting in Firefox 4 Beta 7, Firefox runs in 64-bit mode by default on Mac OS X 10.6 and requires Flash 10.1 or higher. (Firefox 4 on Mac OS X 10.5 or below can run older versions of Flash.)  If you need to use an older Flash version on Mac OS 10.6 for some reason, you can run Firefox 4 in 32-bit mode as a workaround: Close Firefox, open the Applications folder in Finder, control-click the Firefox.app icon, select "Get Info", check the box, "Open in 32-bit mode", close the "Firefox Info" window and then restart Firefox.     
Performance or display issues with certain Flash videos
Flash hardware acceleration can cause performance or display issues with certain Flash videos (e.g., high CPU usage, system instability, hangs, white or green screen, double image split by horizontal green line, no full-screen video, distortion). To work around this problem you can turn off hardware acceleration in your Flash Player display settings:
Note: Starting in Flash 10.1, Flash uses enhanced hardware acceleration (H.264 video hardware decoding) on Windows and Mac systems with supported video cards and drivers (read this for details). Updating your graphics drivers many also help.    
Flash videos stop after 2-3 seconds
YouTube or other Flash videos may initially load but stop playing after 2 or 3 seconds. This is a bug in the Flash plugin which should be fixed in Flash 10.  As a workaround, restart the browser or look through bug 436686 for other solutions, such as using the Flashblock extension to select those videos you wish to play while blocking out other Flash content  or, in Firefox 3 and SeaMonkey 2, by disabling and re-enabling the Flash plugin in the Add-ons manager and then reloading the webpage with the Flash content.  Read the forum topics here and here and this Firefox Support article for more information.
No audio when Flash video originates from an external site
A Flash video originating from an external website may be embedded on a web page, such as a blog that embeds a video from YouTube. If the audio fails to play, open the Flash Player Settings Manager and select the Global Storage Settings Panel. An Adobe Flash Player Settings Manager applet will load as a Flash object. Check the box, “Allow third-party Flash content to store data on your computer” then try the video again. 
Flash audio not working - Windows
First check your Windows Volume Control to make sure the sound isn't muted or turned down. On Windows Vista, open a web page in your Mozilla application that should have sound, then click the volume icon in the Windows task bar, click on "Mixer" and check the Volume Mixer per-application settings. 
Flash audio can also fail in Internet Explorer as well as Firefox and other Mozilla browsers if your Windows audio driver is outdated, if the default device for sound playback does not match the installed audio card, or if the registry key
Flash audio may also fail or may only play while you are rolling over the play buttons, if you have too many Flash audio players loaded (the limit seems to be around 15). Note that this is a bug in the Flash plugin for Firefox and other Mozilla browsers that should be fixed in Flash 10; it does not affect Internet Explorer, which uses the Flash ActiveX control. If updating to Flash 10 does not resolve the issue, try the same workarounds given above (restart the browser; disable and re-enable the Flash plugin; use the Flashblock extension to selectively load Flash content).  
Websites ask you to install or update Flash when Flash 10 is installed
Some websites will ask you to install or update Flash when you have the Flash 10 plugin installed. This is not a problem with your Mozilla browser or your Flash plugin, but with certain websites that misidentify your Flash version (e.g., Flash 10.0 as Flash 0) due to a faulty detection scheme (one digit before the decimal point). Contact the website and ask them to correct the problem or, on Windows, you can try the site in Internet Explorer, since the Flash ActiveX detection may work even though the plugin detection does not.  
Problematic extensions can interfere with Flash content, such as those listed below (this list is not complete). You can selectively disable your extensions in the Add-ons Manager to see if one of them is causing the issue (restarting in Firefox or SeaMonkey Safe Mode will disable all extensions). You can also test the problem site using a newly-created profile.
Amazing Media Browser
RealPlayer Browser Record Plugin
User Agent Switcher
ActiveX plugin conflict
Note: The ActiveX plugin was added to the Mozilla Blocklist on June 7, 2012 and should be disabled by default.
The Mozilla ActiveX plugin can conflict with the Flash plugin and cause errors when visiting sites with Flash content, including:
In Firefox 3 (or SeaMonkey 2) and above, you can disable the ActiveX plugin via "Tools -> Add-ons (or Add-on Manager) -> Plugins"; otherwise, exit the browser and look for the file "npmozax.dll" in the installation directory plugins folder. If found, rename the file "Xnpmozax.dll" to disable the ActiveX plugin. If Flash content plays correctly afterwards, remove the ActiveX plugin.
QuickTime plugin takes over Flash
Note: This issue should no longer exist in QuickTime 7.3.1 or above. See the QuickTime article for help updating QuickTime for Windows. Mac users can update via the OS X Software Update feature. After updating, you may also need to delete the "pluginreg.dat" file from the profile folder location  
If you see a QuickTime symbol with a question mark where the Flash content should be, it means that QuickTime has taken over Flash content; in particular, the application/x-shockwave-flash MIME type for Flash media (.swf extension). To see which plugins are being detected and the MIME types handled, enter about:plugins in your Mozilla browser Location Bar. Note: For security reasons, the Flash media handler is disabled starting in QuickTime 7.3.1 so this should not be an issue in current QuickTime versions.  If you are using an old version of QuickTime (before 7.3.1):
If the problem persists, remove the file "pluginreg.dat" from the "Firefox" or "Mozilla" folder in the profile folder location. Important: On Windows, also open the Mozilla Suite/SeaMonkey or Firefox <installation directory>\plugins folder (e.g., C:\Program Files\Mozilla Firefox\plugins) and remove all "npqtplugin.*.dll" files found. Your browser will then detect the QuickTime plugins based on the MIME types selected in your QuickTime Preferences. 
Illegal Operation in Plugin error
An Illegal Operation in Plugin error referring to "Shockwave Flash" can result from an ActiveX plugin conflict (see above) or for other reasons, including malware, as described in bug 328474 and this forum topic.
Not detected due to disabled plugin scan for PLIDs - Windows
Plugin scanning explains how to prevent your Mozilla browser from detecting plugins installed in directories specified in the Windows registry for PLIDs by setting the plugin.scan.plid.all preference to false. The problem with disabling the PLID plugin scan is that it disables all plugins that are detected this way, including Flash. A workaround is to copy NPSWF32.dll and flashplayer.xpt from C:\WINDOWS\SYSTEM32\Macromed\Flash to the installation directory plugins folder.  .
An alternative solution is to re-enable plugin scanning for PLIDs and disable the unwanted plugins using the Add-on Manager instead. See here for more information.
Flash files do not play
Some users find that direct links to Flash files (.SWF) will not open automatically in the browser, but instead cause the file Opening dialog box to appear, asking whether to save the file or to open it in a suitable application. This can happen when the server does not send the Flash file with the correct MIME type. In other cases, the Flash installation may be faulty, which can be fixed by uninstalling and reinstalling Flash. In Firefox 3 or SeaMonkey 2, the issue can also occur if the (Helper) Applications entry for Flash content is set to "Always ask" instead of "Use Shockwave Flash". See Opening files using plugins for more information.