The following is from Firefox's knowledge base :
If Firefox processes that remain in memory at exit are a recurring problem, try these solutions:
* Problematic extensions, including AVG Safe Search, PicLens, and Skype Extension for Firefox, can cause the problem, which can be fixed by disabling or uninstalling the extension. See Troubleshooting extensions and themes for more information.
* 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:
o If you use ZoneAlarm on Windows Vista, uninstall it and use another firewall product (or use the Windows Firewall).
o If you use Avast! Antivirus, switch to another antivirus product if you also have ZoneAlarm installed.
o 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.