A Mozilla tem um artigo muito detalhado sobre a solução de problemas trancados no Firefox, para seus questão:
Hang at exit
Sometimes when you close Firefox, it may stop responding and remain in memory, even though no Firefox windows are open. This can prevent Firefox from working properly the next time you open it or you may see a "Close Firefox" dialog box with the message, Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.. You must then end all Firefox processes or restart the computer before you can reopen Firefox. (See Firefox is already running but is not responding for other causes and solutions.)
If Firefox processes that remain in memory at exit are a recurring problem, try these solutions:
A problematic extension can cause the problem, which can be resolved by disabling or uninstalling the extension. For information on diagnosing and fixing problems caused by faulty extensions, see the Troubleshooting extensions and themes article.
Sometimes Java applets can cause the Firefox process to persist after exit. Try updating Java to the latest version or, if you don't need Java, disable the Java plugin by clearing the "Enable Java" checkbox in Firefox (see Using the Java plugin with Firefox for details).
Windows:
Using certain Internet security software is reported to cause the issue on some systems:
- If you use ZoneAlarm on Windows Vista, uninstall it and use another firewall product (or use the Windows Firewall).
- If you use Avast! Antivirus, switch to another antivirus product if you also have ZoneAlarm installed.
If you use Norton 360 2.0, make sure that you have all available Norton updates (for more help, contact Norton Support) and that you have followed the instructions in the Configuring Norton 360 article.
If you have upgraded Comodo Internet Security from a version prior to 3.10, the Comodo Safe Surf component from the previous version can cause Firefox to hang at exit. To resolve the issue, you can uninstall Comodo Safe Surf via Windows Control Panel, as its function is now integrated into the current Comodo application.