Firefox is my primary browser.
It's also the one that frustrates me the most (obviously, I don't even bother with IE).
A few interface bugs persist, and I doubt they will ever be fixed. One such annoyance is how firefox handles multiple saved logins for the same site. Load that site and the logins take a while to load. Plus, you have to click (or start typing and auto-complete) to select the login you want.
The Secure Login plugin fixes this, by gifting Firefox with Opera's superior password handling. Now there is always a single path to login (use the keyboard shortcut you select, or click the key icon next to the address bar). Multiple logins are ready right when the page loads, and there is no fuss about getting to the login you want.