The first thing to do is check if your plugins are up to date or need to be updated,
by using the plugin checker from the Mozilla Firefox site :
http://www.mozilla.org/en-US/plugincheck/#
Also ensure your extensions are up to date or need to be updated,Firefox usually
does this automatically every so often.A recent plugin,extension or theme you added
into Firefox could also be the cause if it isn't compatible with your version.
Where available Firefox also uses hardware acceleration,where some aspects of a
site you visit is handled by your graphics card in your computer.This may be a cause
of the Firefox crash problems.Try turning this feature of to see if it fixes the problem.
To the top left of Firefox,click the Firefox button and select options.Select Advanced
and then the General tab.Uncheck Use hardware acceleration when available.
Click OK to finish.For more help on troubleshooting visit this page from the Mozilla
Firefox site :
http://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems#w_turn-off-hardware-acceleration
Hope that helps you,Tidus