Firefox already features a password manager, but if that's not enough for you, then you may want to give Sxipper a try.
Sxipper is a Firefox add-on that managers all the usernames and passwords you enter on many different websites on a daily basis. Every time you enter a username and password, Sxipper will offer to remember the data - just like Firefox does – so that next time you visit that site, you don't have to type the same information again.
The difference is that Sxipper is able to fill in complete forms (not just the username-password couple) and can also store different data for different users. I especially liked the way stored data is presented, as an overlay window in which you only have to click on your username to log in to the website.
The only drawback in Sxipper is that it doesn't work with websites you have previously told Firefox not to save passwords for. In that case you'll have to go to the browser's settings to delete them from the list.
Sxipper is a useful add-on that makes user authentication easier and faster on any website you visit.