If starting in Safe mode doesn't fix it then try this: It is possible that there is a problem with t

If starting in Safe mode doesn't fix it then try this:

It is possible that there is a problem with the files key3.db and signons.sqlite that store the encrypted names and passwords in Firefox.
Rename the files key3.db and signons.sqlite in the Firefox profile folder.
You can add .old to the file names (key3.db.old and signons.sqlite.old) or move them to another folder to make it possible to undo the action.
You need to set a new Master Password after renaming or deleting key3.db and all currently saved passwords are lost.
If that has worked then you can remove the renamed files that are no longer needed.

See:

"Troubleshooting" in http://kb.mozillazine.org/Password_Manager