Try this out, it may be help full :- https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems

use safe mode for starting fx browser

I have same. Vista 7 and er...sometimes Firefox. I have the latest Firefox installed hoping that it wold stop that Mozilla Firefox (not responding) cropping up... ... My mistake. Since up(lol)grading to the latest that error is almost permanantly etched into my screen. It is DREADFUL. Even at launch I can get that message. I can NEVER get past 35-40 mins before it will crop up and hang the PC. I get that and its oft companion "Plug in busy stopping you having fun" etc I have written to Mozilla twice3 (only to be ignored) that this is not OUR error, but THEIRS. Basically your browser falls over. As for solutions - sorry - I will not be fobbed off with FAQs (which never cover one's particular prob) and the "solution" of "Oh, just RESET Firefox" does NOT work. It won't work because all I am doing is rebooting a faulty browser. So whilst having to go back to IE will Firefox be actually FIXING their software so that the NEXT upgrade can actually SUSTAIN a browsing session that could ooo maybe go over an hour? Currently I have the Task Manager open permanantly so as to flag "END TASK" next to "firefox is not running...."

Tried Safe Mode with no improvement. Uninstalled FF, rebooted computer, reinstalled FF. All problems now gone.