Any chance that you once had Firefox 4.0b7 installed?

Create a new profile as a test to check if your current profile is causing the problems.
See Basic Troubleshooting: Make a new profile

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

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

Hi - I was using 3 and that was when it crashed. I uninstalled it and installed the beta of version 4 but no change.

Tried creating a new profile and it didn't work. Same problem with the Crash Reporter appearing and firefox not launching.

Do a clean (re)install:

• Download a fresh Firefox copy from http://www.mozilla.com/firefox/all.html and save the file to the desktop.
• Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.
• Don't remove personal data if you uninstall the current version.
• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.

See:

• Troubleshoot and diagnose Firefox problems

Thank you. That worked. I have been able to reinstall and get Firefox working. Can I just check how can I get my old bookmarks to transfer across? Thanks.

Read this. http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox