I like Firefox. A lot. Even when it was named Phoenix.
I even use it at work (using PortableApps.com).
But some sites (ok, only StackExchange sites) I visit I get multiple popups asking for my user name and password. Because I’m behind a firewall I have to enter my credentials, no big deal. But they don’t work! So I have to hit Cancel. Not just once, but, like, 4 times!