Shammah,
A good first step is to make sure you have all of apple's software updates applied. To check this, simply click on the apple logo in the top left, and choose "software update." Install any available updates.
Once this is done, run a disk permissions repair by using disk utility (applications --> utilities --> disk utility).
Once you open disk utility, click on your hard drive on the left hand side and choose "repair disk permissions" in the bottom right.
If it's still not working after that, you can always trash your firefox preferences file. If it gets to that point, let me know and I'll walk you through it.