From MozillaZine Knowledge Base
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Mozilla applications are regularly updated for security and stability; however, Mozilla does not provide updates for plug-ins. Plug-ins can cause crashes and hangs and may contain security vulnerabilities. Visit Adobe's website for information on security and stability issues affecting Adobe Flash.
Enabling and disabling Flash
Installed plugins, including Flash, can be disabled and enabled in the Add-ons Manager (see Managing installed plugins for details). To disable or enable Flash:
Note: On Windows, the Flash plugin is detected via plugin scanning. If you have installed the Flash plugin but it does not appear in the Add-ons Manager, open about:config and make sure that the preference plugin.scan.plid.all has not been set to "false" (if it has, right-click on it and select Reset to set it back to "true"). 
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 the following:
In Flash versions 11.2 and above, the file name includes the plugin version. For example, for Flash plugin version 220.127.116.11 on Windows, the plugin file name would be "NPSWF32_11_2_202_235.dll". In older Flash versions, for example, plugin version 10.3.183.18 on Windows, the file name would simply be "NPSWF32.dll".
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. Note that recent installers are only helper applications that download the actual application from the Adobe site, thus they are smaller in size than previous downloads.
You can download the full installer for your operating system, without any bundled software, from the Adobe Flash Player Distribution page. 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.  Archived Flash versions can be downloaded here.
After downloading the Flash installer, completely close your Mozilla application and run the installer. 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. 
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 (PFS) can download and install Flash for you (see this article for details) but it may offer an outdated version. Note: Mozilla may either remove the PFS or change the process for downloading missing plugins, at some future date.   
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.
Important: 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.
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, and leave the Flash Player ActiveX installed, just remove the "Adobe Flash Player Plugin" from the Windows Control Panel list of installed programs (see below).
Uninstall using Windows Control Panel
Flash 11, Flash 10, and Flash 9.0 r45 or 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). If you see an entry for "Adobe Flash Player ActiveX", this is the Flash ActiveX control needed for Internet Explorer and AOL. Flash Player ActiveX may also be required for other software to work properly (see above). Note: It's possible to have two different versions of Flash Player installed; for example, Adobe Flash Player 11 ActiveX for Internet Explorer and Adobe Flash Player 10 Plugin for Mozilla browsers.
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 the Flash Player 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 preference 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.
Warning: Installing an old 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.  Also, Mozilla adds outdated Flash plugin versions to its Add-ons Blocklist as a security measure. In some cases, this may may result in "infobar" notification in the browser. In other cases, outdated Flash plugins may be disabled in the Firefox or SeaMonkey Add-ons Manager or, starting in Firefox 17, may require Click to play activation. 
If your problem isn't listed below:
Browser hangs playing Flash videos
If Firefox or SeaMonkey hangs (locks up, freezes, or stops responding) when you play Flash videos, try disabling hardware acceleration in Flash settings or, on Windows Vista and above, try disabling protected mode in Flash. See Performance or display issues with certain Flash videos and Flash Player 11.3 Protected Mode - Windows (below) for details.
Flash Protected Mode issues on Windows Vista and above
Starting in Flash 11.3 the Flash Player plugin used in Firefox and SeaMonkey runs in a Protected Mode by default on Windows Vista and above. This security feature has been causing problems for some Firefox or SeaMonkey users since it was introduced in Flash 11.3. See Flash Player 11.3 Protected Mode - Windows (below) for suggested solutions and workarounds, which also apply to Flash 11.4 and above.
Flash Player 11.3 Protected Mode - Windows
The suggested solutions and workarounds in this section apply to Flash 11.3 and above
Starting in Flash 11.3 the Flash Player plugin used in Firefox and SeaMonkey runs in a Protected Mode on Windows Vista and above. When Flash is running in Protected Mode, you will see two "FlashPlayerPlugin_<version>.exe processes in Windows Task Manager (e.g., “FlashPlayerPlugin_11_300_271.exe”). This security feature can cause crashes, freezes, slow or busy script warnings, An error occurred. Please try again later messages at YouTube, Adobe Flash "plugin has crashed" or "stopped working" errors, loss of browser focus, large numbers of empty FAP****.tmp files or other problems after visiting websites that use Flash. Mozilla and Adobe are working on these issues, so make sure to keep Firefox, SeaMonkey, and Adobe Flash updated to take advantage of bug fixes.        
As a workaround, you can disable Flash Protected Mode (see below) or you can downgrade the Flash plugin to a version previous to 11.3, such as Flash 11.2 or 10.3; however, older Flash versions have security vulnerabilities and may be blocklisted. To downgrade Flash, uninstall the Flash plugin via Windows Control Panel "Programs and Features" (or use Adobe's uninstaller), and then install the older plugin version. See Adobe's How do I revert to a previous version of Flash Player? page for details. Also note that the latest Flash 10.3 plugin installer was previously recommended because it included all known security patches. As of July 9, 2013, however, Flash 10.3 will no longer be maintained.  
Important: Before downgrading Flash or disabling Flash Protected Mode:
Disabling Protected Mode in Flash 11.3
To disable (or re-enable) Protected Mode in Flash 11.3 and above, use one of these methods:
To disable Protected Mode, use a text editor to add the line ProtectedMode=0 to the Flash "mms.cfg" file, located in one of these locations: 
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
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, sound plays but no video). As a workaround, you can turn off hardware acceleration in your Flash Player display settings. 
This may only be a temporary solution, as long as the Flash setting to turn off hardware acceleration is stored as an LSO (Flash cookie), as reported for Flash 10.3 in this Adobe forum thread. Clearing cookies in Firefox deletes this type of cookie, as will using an extension such as BetterPrivacy.     
On Windows, you can also disable hardware acceleration by adding the line DisableHardwareAcceleration=1 to the Flash "mms.cfg" file, as suggested in this Adobe forum thread. The mms.cfg file is located here:
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
Note: If Flash 11.3 is installed, first read: Flash Player 11.3 Audio Update at Adobe's forum.
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). On Mac OS in Firefox 4 and above, this process is named "Firefox Plugin Process (Shockwave Flash)". See Plugin-container and out-of-process plugins for more information about this crash protection feature.
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.
The plugin for this content has been disabled
If you see a box with the message, This plugin is disabled. Manage plugins... or 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. 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. 
Plugin Check reports outdated Flash has been disabled when the Flash plugin is not installed
Due to a bug, Mozilla's Plugin Check page will display the message, Missing FLASH? For your safety, Firefox has disabled your outdated version of Flash. Please upgrade to the latest version. even though the Flash plugin is not installed. (screenshot)