If you haven’t made the jump yet, there are a few files you may want to make sure you back up before going from Firefox 2 to Firefox 3.  The first would be your password files.  These files are located in your profile folder. 
Backup the Firefox files called:

signons.txt
signons2.txt
key3.db

Copy and past them into the place you want to back them up to, and you are done.  To restore them, make sure you don’t have Firefox running and then go [...]