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.
Thank you for the suggestion. I'll have to try the Disk Utility when I find my mac installation disk at home as I'm away at college right now. Thank you for the tip and I'll let you know how it goes if I get to try it out soon.
I tried to delete firefox and install the newest version but it didn't work.