Try using Disk Utility to do a
Disk Repair, as shown
in this link, while booted up on your install disk.
You could have some directory corruption. Let us know what errors Disk Utility reports and if DU was able to repair them. Disk Utility's Disk Repair is not perfect and may not find or repair all directory issues. A stronger utility may be required to finish the job.
After that
Repair Permissions.
No need to report
Permissions errors....we all get them.
If that doesn't do it, download the newest version of Firefox, trash your old one, and install the new one.
DALE