Create a new profile as a test to check if your current profile is causing the problems.

See "Basic Troubleshooting: Make a new profile":

• https://support.mozilla.com/kb/Basic+Troubleshooting#w_8-make-a-new-profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins" in case there are still problems.

If that new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

See:

• http://kb.mozillazine.org/Firefox_crashes
• https://support.mozilla.com/kb/Firefox+crashes

If you have submitted Breakpad crash reports then post the IDs of one or more Breakpad crash reports (bp-xxxxxxxx-xxxxxxxxx-xxxx-xxxxxxxxxxxx). You can find the IDs of the submitted crash reports on the about:crashes page. You can open the about:crashes page via the location bar, like you open a website.

See:

• http://kb.mozillazine.org/Breakpad (Mozilla Crash Reporter)
• https://support.mozilla.com/kb/Mozilla+Crash+Reporter
• https://developer.mozilla.org/en/How_to_get_a_stacktrace_for_a_bug_report

Do you have Trusteer Rapport installed? If so, disable it.

   * Windows: "Start->All Programs->Trusteer Rapport->Stop Rapport"
   * Mac: "System Preferences -> Other -> Rapport -> Stop Rapport" 

For more help you can ask their helpdesk.
http://www.trusteer.com/support/en/report-problem

http://kb.mozillazine.org/Firefox_crashes#Trusteer_Rapport

Roboform is also causing Firefox 8 to crash upon startup. Install the latest 7.6.2 version of Roboform.
http://www.roboform.com/support/news

Update Roboform. That has been fixing this for many people, including me!  :)