This is a problem known to Firefox. The following is from the Firefox support pages, which can be found at http://support.mozilla.com/en-US/kb/Firefox+is+already+running+but+is+not+responding.
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 the Java plugin, you can disable it from the Plugins panel of the Firefox Add-ons window. For more information, see Using the Java plugin with Firefox.