Originally Posted by Redlemon

Quit Firefox, go to your profile folder, and trash the files key3.db and signons.sqlite. Firefox will create new ones on restart. It is possible you have some corruption in one of those files. Obviously, you'll have to recreate all of your passwords after doing this.

Another possibility is that you have set your Privacy options to clear passwords on exit.

If you need more instructions that what I just wrote, let me know.