This is most likely an add-on issue.

Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• On Windows you can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac you can open Firefox 4.0+ in Safe Mode by holding the option key while starting Firefox.
• On Linux you can open Firefox 4.0+ in Safe Mode by quitting Firefox and then going to your Terminal and running: firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

Please report back soon.

If the issue still occurs after following my above steps, this may be a malware issue.

Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Did this fix your problems? Please report back soon.

Right on--thanks for the response. Safe Mode showed no problem so I removed all extensions and added them back one-by-one, testing each one as I went. The one extension which seemed to freeze Firefox upon exit was AdBlock Plus 2.4 so I'll have to do without that one but with the others reinstalled, Firefox is working just fine. Thanks again for your help.

Thanks for your reply. The issue turned out to be with the extentions so I removed the single extension (AdBlock Plus 2.4) which seemed to be causing the problem. I appreciate your speedy reply and especially your suggestions for testing malware which can always pose a threat and for which I should be on the lookout more vigilantly generally.

Glad to hear that your issue has been resolved.
Thank you for using the Mozilla support forum.