I would review your add-ons, and see if any are causing any problems.
Starting Firefox in its Safe Mode might be a good way to check. That starts FF will all add-ons disabled. If that halts the problem, then you can decide how to check which add-on is causing the problem.
Firefox in Safe Mode - In Windows go to Start > Run, type in firefox -safe-mode and click OK. The space between firefox and -safe-mode is important.
If that doesn't solve the problem, then may be your Firefox profile has become corrupt. See the post # 2 in John Wilkinson's Sticky Post for options how to repair Firefox;
I'm having a problem with the Firefox browser not closing properly.
I click on the X in the top right to close the browser and it close but sometimes the hard drive light is staying on. If I bring up the task manger it shows that the firefox.exe is still running.
I then have to end the process in the task manager or I can't open the browser.
Even when I do that it either brings up the last webpage I was at or says that its embarrassed and wants to know if I want to bring up the last website I was at or start a new session. Turning off the computer doesn't help this either.
When I turned on the computer this morning it asked me if I wanted to go to the last website I was at before I turned off the computer or start a new session. Anyone have an idea whats causing this????