Just wanted to post a solution I found to the highly annoying problem of GMail crashing Firefox when you close a GMail tab or move away from GMail.
If you google for this error, you’ll see most sites agree, including the Mozilla team, that the GMail Talk Plugin is causing this error. The solution is to disable Google Talk. Just look under your Add-ons and uninstall GoogleTalk.
The only problem is I don’t have Google Talk installed. So now what?
To find out that GoogleTalk is definitely the cause, you’ll first need to check the firefox crash report to make sure it is reporting GoogleTalk or UserCallWinProcCheckWOW. You need to report the error using the Mozilla Crash Reporter (the box that pops up on a crash that asks if you want to report the error). Report it and then look at the crash log. If you don’t report the error, it will not appear in your crash log.