Sounds like it is hanging when you exit and later try to restart Firefox.

• How to stop the running Firefox process
  • In Task Manager, does firefox.exe show in the Processes tab?
  • See: http://kb.mozillazine.org/Kill_application
  • Windows 7 users: http://www.techrepublic.com/blog/window-on-windows/reap-the-benefits-of-windows-7s-task-manager/2576
• What may cause Firefox to hang at exit
  • http://support.mozilla.com/en-US/kb/Firefox+hangs
  • http://kb.mozillazine.org/Firefox_hangs
  • https://support.mozilla.com/en-US/kb/Firefox+is+already+running+but+is+not+responding
• You may need to try Firefox Safe Mode
  • You may need to use Safe Mode to locate the problem:
    • See:
      • https://support.mozilla.com/en-US/kb/Safe%20Mode
      • http://kb.mozillazine.org/Safe_Mode
    • Firefox 4 & 5 users, hold the Shift key while clicking the Firefox 4 desktop icon to enter Safe Mode or, when Firefox 4 or 5 is open you can use the menu item Help > Restart with Add-ons Disabled (Firefox immediately shuts down and then restarts)
    • Firefox Safe Mode is a diagnostic mode that disables Extensions and some other features of Firefox.
      • In Firefox 4 & 5, Safe Mode also disables Plugins and Hardware Acceleration.
      • If you are using a theme, switch to the DEFAULT theme: Tools > Add-ons > Themes before starting Safe Mode (In Firefox 4, Tools > Add-ons > Appearance). See: http://support.mozilla.com/en-US/kb/Using%20themes%20with%20Firefox#w_managing-themes-and-personas
      • When entering Safe Mode, do not check any items on the entry window, just click "Continue in Safe Mode".
      • Test to see if the problem you are experiencing is corrected.
    • If the problem does not occur in Safe-mode then disable all of your Extensions and Plug-ins and then try to find which is causing it
      1. by enabling one at a time until the problem reappears.
      2. You can also enable a few at a time (3-5) or half of them, if you wish, keeping track of what you just disabled/re-enabled to see if the problem re-occurs; this will help narrow down the possibilities if you have a large number of add-ons.
      3. You MUST close and restart Firefox after EACH change via File > Restart Firefox (on Mac: Firefox > Quit).
      4. You can use "Disable all add-ons" on the Safe Mode start window.
  • See the following for more information
    • http://support.mozilla.com/en-US/kb/Troubleshooting+extensions+and+themes
    • http://support.mozilla.com/en-US/kb/Troubleshooting+plugins
    • http://support.mozilla.com/en-US/kb/Basic+Troubleshooting

The following may not completely eliminate having to terminate Firefox through the Windows Task Manager but it will come very close.

Plug-in and tasks started by Firefox may continue after attempting to close Firefox The "X" in the upper right-hand corner closes the Window (same as Ctrl+Shift+W) but even if it is the last Firefox window, it does not necessarily close Firefox .

The only proper way to exit Firefox is to use Exit through the File menu, not the "X" in the upper right corner of last Firefox window. In the Beta that would be Alt+F then X

• Firefox hangs | Troubleshooting | Firefox Support
  http://support.mozilla.com/en-US/kb/Firefox%20hangs#w_hang-at-exit

Use the Windows Task Manger to remove all running firefox.exe in the "Processes" tab of the Windows Task Manager, then restart Firefox.

very infrequent -- If Firefox will still not start, remove the parent.lock file from the profile which is created each time Firefox is started to prevent other Firefox tasks from running, see
http://kb.mozillazine.org/Profile_in_use#Remove_the_profile_lock_file

Avoiding Problems with close/restart choose either extension

Use to close and restart Firefox after enabling or disabling an extension, switching to a new theme, or modifying configuration files, then you don't have to worry about delay or have to look in the Task Manager to see if Firefox is closed yet.

• "Restartless Restart" extension for Firefox 4.0+ only (2 KB download )
  uses keyboard shortcut "Ctrl+Alt+R" or a file menu
  https://addons.mozilla.org/firefox/addon/249342/

Windows 7 users can use Ctrl+Shift+Esc to get to Task Manager quickly, and can include the "Command Line" column in the Processes tab so you can identify how each firefox.exe was started and which ones to terminate if not all of them. Especially for Beta testers running multiple versions of Firefox at the same time.

Thank you very much for your help, both responses were useful. I just have to figure out which plug-in is causing the problem.

You are welcome.

So that other users can find solutions to previously asked Firefox support questions, please take a couple of minutes to return to the forum, sign in and

1. click "Solved It" next to the reply above that you found MOST helpful in solving your question
2. you can click "Helpful" next the OTHER reply above, if you wish.

Thanks.

Running in safe mode did the trick. I quickly identified that Symantec IPS 3.0 was updated July 08, 2011. Enabling this add-on results in Firefox 5.0 hanging in memory after exiting every time (requiring that I close it using the Task Manager).

Problem fixed. Symantec forum has following fix for others with same problem (compatibility issue with Norton Toolbar). See http://us.norton.com/support/kb/web_view.jsp?wv_type=public_web&docurl=kb20100720112959EN_EndUserProfile_en_us