Unable to save or download files
From MozillaZine Knowledge Base
If you are unable to save or download files in Firefox, try these suggestions:
Choose a different download folder
Firefox will automatically download files to a default location (e.g., to the Desktop or, depending on your operating system, to a special "Downloads" folder) unless you've selected a different download folder or you've set Firefox to ask where to save every file.    If downloads fail or if Firefox hangs downloading files it may be related to the download location; for example, the download folder may contain a lot of images , may be corrupt or inaccessible  or the folder may no longer exist.  
To choose a different download folder, go to "Tools -> Options" and under "General / Downloads" ("Main / Downloads" in Firefox 3.5), select "Save files to" (if not already selected) and click the "Browse" button ("Choose" button on Mac OS X).
Note: In cases where the "Ask me where to save files" option has been selected, the problem may be related to the last-selected download folder. Instead of choosing a download folder you can try resetting the browser.download.lastDir preference in about:config (see below).
Reset download folder
Firefox may have lost track of which folder to use for the download directory. Symptoms would include "File - > Save Page As" and the "Save Image As" and "Save Link As" right-click context menu options not functioning or nothing downloading when you select the "Save" option in the Firefox "Opening" dialog box  
The solution may be as simple as choosing a different download folder, then going back and re-selecting your preferred download settings in Firefox Tools -> Options under "General / Downloads" ("Main / Downloads" in Firefox 3.5). For example, if "Always ask me where to save files" is selected, select "Save files to" and choose a download folder, then go back and re-select "Always ask me where to save files". 
If you still have problems you can restore the default Firefox download folder settings as follows:
Enter about:config in the address bar. A list of preferences will appear. Scroll through the list ( or type browser.download in the Filter box) and reset any of the following preferences that have a user set status (right-click on the preference and click "Reset"):
You can then change back to your preferred download folder settings, if you wish, in your Firefox options/preferences.
If you'd rather just reset all Firefox preferences to the default values, you can delete (or rename) the prefs.js file in your Firefox profile folder or use the Safe Mode option, "Reset all user preferences to Firefox defaults". See Resetting preferences for more information.
Choose a valid download folder in Safari Preferences - Mac
Whenever you attempt to download a file, you may see the error, <filename> could not be saved, because an unknown error occurred. (Save As.. will still work). To fix this problem on Mac OS X 10.4 and earlier, open Safari Preferences and check the "Save downloaded files to:" setting. If this is pointing to an invalid directory, it will cause downloads in Firefox to fail. Select a valid directory, such as your Desktop. Note that Safari no longer controls the default download location in Leopard (10.5).  
A user reported in this thread that also changing the location using the old, unsupported Internet Explorer for the Mac solved this problem. If you still have a copy of that application, you might try this approach.
Clear the download history
In the Firefox (Tools) menu, go "Downloads" and click "Clear List". This will remove the stored history of downloaded files in Firefox, not the actual files that are stored on your computer.
If clearing the list doesn't help or it causes Firefox to hang or crash (or if the Downloads window is already empty), you can manually clear the download history, as follows:
If clearing the download history resolves the issue, you can set Firefox to not save a download history in your "Privacy" options for History. Click "Tools -> Options -> Privacy" and set "Firefox will:" to "Use custom settings for history" and then uncheck "Remember download history". However, this will automatically clear the Downloads window, leaving it empty after each download is completed. As an alternative, you can set Firefox to clear the download history whenever you close Firefox:
Change or reset actions for file types
Firefox may be set to handle certain types of files with an automatic action and sometimes it is not what you want or may result in errors. For example, Firefox may automatically open a certain file type with an application or plugin, when you would rather save the file. Check for entries related to the problem file type via "Tools -> Options -> Applications" and change the action. For more information, see File types and download actions.
If you see this error, delete (or rename) the mimeTypes.rdf file in the Firefox profile folder, as explained above. This will resolve an issue in which Firefox tries to open executable (exe) files, which causes this error to occur.  
Delete corrupt plist file - Mac
Remove download manager plugins - Windows
Download manager plugins can interfere with Firefox downloads if the download manager is uninstalled or otherwise stops working. Such plugins include:
Information about other download managers can be found at PluginDoc. If you find any of these plugins, remove the plugin files from the Firefox plugins folder (usually C:\Program Files\Mozilla Firefox\plugins) or disable the plugins via "Tools -> Add-ons -> Plugins". See Issues related to plugins for more information.
Problematic extensions can cause download or file-saving issues, such as nothing happening when you click a download link  or a missing "Save File" option in the "Opening" dialog  (in the latter case, press the "Enter" key as a workaround). To see if an extension is causing your problem, disable it from the Add-ons manager ("Tools -> Add-ons -> Extensions") or via Firefox Safe Mode. Since certain extensions also modify preference settings, you may also need to reset those preferences via about:config (or reset all preferences).
Temporarily disable antivirus or update security software
You may see the above message when you click a link to download an executable (.exe) file. This problem can be caused by a conflict with your antivirus scanner and a firewall product such as Comodo Firewall (see this Comodo forum topic for details).
Some firewalls continue to be active when disabled in the user interface, so disabling the firewall may be ineffective. As a workaround, temporarily disable your resident antivirus scanner when downloading an executable file, then re-enable it when the download is complete.
Note: PC Tools ThreatFire (formerly Cyberhawk) can also cause the message, <filename> could not be saved, because an unknown error occurred when downloading certain types of files.  If you cannot work around the problem or resolve it by updating the software, consider using another security product.
Disable virus scanning in Firefox preferences - Windows
In Firefox 3 and above, Firefox launches your installed anti-virus program to scan files when the download completes.  In some cases, this causes a substantial delay or hang while the file is scanned.   In other cases, the file is not saved after the scan completes.    You can disable this feature by setting the preference browser.download.manager.scanWhenDone to false in about:config.  Note: If the Download Manager appears to be stuck scanning for viruses, try closing and re-opening the Downloads window instead of disabling the preference.  
Enable downloads blocked by Security Zone Policy - Windows
In Firefox 3 and above, Firefox honors your Windows Security zone settings for downloading and launching potentially unsafe files from the Internet. Based on your settings, downloads of executable files (e.g., .exe or .msi) may fail and the Downloads window may contain the following message under the file name:
In Firefox 3.5 and above, the Downloads window may simply show "Canceled" after the file name.
This can occur if you have customized your Internet Options Security settings for the Internet zone, if the option, "Launching applications and unsafe files" is set to "Disable".   Depending on your Windows version, this can happen if the Internet zone Security level is set to "High" (e.g., on Windows XP).  You can resolve this issue as follows:
Bypass Windows Security Policy check
Starting in Firefox 3.0.6 and in Firefox 3.5.x you can bypass the Security Zone Policy check. by using about:config to add a new Boolean preference, browser.download.manager.skipWinSecurityPolicyChecks and set it to true. .
This preference was removed in Firefox 3.6.   An alternate solution is to reset your Windows Internet security settings, as explained below. In Firefox 4 and above, you can bypass the Windows security policy check by setting the preference browser.download.manager.scanWhenDone to false in about:config. 
Reset system Internet security settings - Windows
If your Windows Internet Options, Security zone setting for "Launching applications and unsafe files" is set to "Disable", Firefox 3 will block downloads of executable files or, in Firefox 3.5 and above, it will cancel the download and show Canceled - <domain name> in the Firefox Downloads window (see above).  To resolve this problem, use one of the following methods.
Method 1: Reset your Security settings for the Internet zone to the default level. Go to "Control Panel -> Internet Options" (or use "Start -> Run -> inetcpl.cpl -> OK" to open "Internet Properties") then click the Security tab, select the Internet zone and click "Default level" (see this article for a screenshot).  
Method 2: Set a custom level for security in the Internet zone. Go to "Control Panel -> Internet Options -> Security (tab) -> Internet" (as explained above). If security is set to "High", or if you have customized the Internet zone settings, select "Custom level..." and change the single option, "Launching Applications and Unsafe Files" from 'Disable' to 'Prompt (recommended)'.
In some cases, you may need to update, reinstall, or reset Internet Explorer to restore or customize the default Internet security settings.   . To reset all Internet Explorer settings, see Microsoft's articles here and here. For more help, visit a Windows forum such as Microsoft Answers, AumHa Forums or WindowsBBS.com.
Note: If executable file downloads fail from certain sites but you are able to download exe files from other sites, check in "Internet Options -> Security - > Restricted sites -> Sites" to make sure that the problem site is not in the "Restricted sites" list, as the security level for that zone is High by default. Some Security software, including Spybot S&D, add known malware sites to that listing.