Figuring out the cause of a crash can be difficult. If you've tried the steps above and can't get Firefox to stop crashing, the steps below will show you how to collect information about your crash so our volunteers can help you.

   Get the Report ID of your latest crash report:
       You can open Firefox, at least in Safe Mode:
           In the Firefox Location bar, type about:crashes and hit Enter. A page listing submitted crash reports will appear.
           Right-click on the latest crash under Report ID and choose Open Link in New Tab. A tab will open to a page on crash-stats.mozilla.com, saying that "Your report is being processed". When it is finished processing, close the report tab. 
       You can't open Firefox because it crashes at startup even in Safe Mode:
           Click the Windows Start button, and select Run... on Windows 2000/XP or press Windows Key + R on Windows Vista/7. Type in "%APPDATA%\Mozilla\Firefox\Crash Reports\submitted" including the quotation marks and click OK.
           In the resulting folder will be a text file for each crash report you have submitted. Using the View menu, arrange them by date to find out which is the newest file; and double-click on it to open it. 
   Click this special link to ask for help with your crash.
   Note: This link is only valid for help with crashes.
   Click the edit link next to your question title and make it more descriptive of the problem you're seeing.
   Fill out the Details section with as much information you have about the crash. Include any of the steps you completed above, like updating Firefox or testing for the crash in safe mode.
   Copy the long Report ID number you've got above for the latest crash (looks like bp-...) and paste it into the Crash ID(s) section of the form you're filling out.
   Get additional troubleshooting information:
       You can open Firefox, at least in Safe Mode:
           Go to the top of the Firefox window and click the Firefox button. Then go over to the Help menu and select Troubleshooting Information. For Windows XP: Go to the Help menu and select Troubleshooting Information.
           On the Troubleshooting Information page, click the Copy all to clipboard button and then paste that into the Troubleshooting Information section of the form you're filling out. 
   Finally, click the green Post Question button.

Thanks Vishunushaji. That's exactly what I did. The info is under More System Details on the right of my question. My detailed edited field was apparently posted here by the procedure you describe.

JW

Updating the both Firefox and Thunderbird to the latest version seems to have resolved the crash... unsure where it came from but resolved.

I noted the crash ID but have not had time to try to read and understand the info but may as well post it as it may help others.

• Crash ID report : 5809b726-8634-40ab-8c6b-8257a2120124
• Crash Signature: JSObject::getGlobal()
• related bugs
  • Bug 701130 - Firefox 11.0a1 Crash Report [@ JSObject::getGlobal ]
  • Bug 691847 - Firefox Startup Crash @ js::Bindings::getLocalNameArray

See also firefox crashes and latest firefox issues noting sometimes updating firefox and other software does solve problems with crashes. If it is a crash on startup you may need to use another browser to download a fresh firefox install, if you do that remember NOT to delete personal information in the re-install process.