I had to troubleshoot a computer with Firefox 12. Firefox 3 ran fine. I cleared the profile in case that was corrupt, and it did nothing. It would start fine in safe mode.
After troubleshooting for a while I found the issue. Hardware Acceleration was turned on by default, which was causing Firefox to crash. Firefox (FF) worked fine in Safe Mode though, because Hardware Acceleration is OFF in Safe mode.
How to fix:
1) Start FF in Safe mode by holding down Shift while clicking the icon.
2) Go to Tools, Options, Advanced, General.
3) Uncheck “Use hardware acceleration when available”
4) Click “OK” and restart Firefox normally.
This particular computer ran Windows XP, however the issue could crop up in any computer where there is a driver conflict of some sort that caused this.