I had to re-install FF and it no longer is doing this. I use the Xmarks plug-in and it's saving my bookmarks OK but not saving or asking for passwords.
When I re-stored FF Xmarks gave me all my bookmarks and passwords from previously but wont save new passwords anymore.
I'm not sure if I'd had some other plug-in installed last time that I've forgotten about and not installed now or if something has changed with Xmarks or FF.
Any ideas?