Among the more common issues that have had people grousing over the years is that Firefox doesn’t use the Mac OS X keychain to save login information (plus the equivalent gripe on Linux). For most users this isn’t really that big a deal, but admittedly the fact that keychains can be synchronized across computers if you’re a .Mac user has its appeal.
Firefox 3’s new Login Manager provides the nsILoginManagerStorage interface, which you can implement to provide a replacement login information storage model for Firefox.
We now have documentation for how to do this sort of thing. Enjoy!