So I've just finished making the switch for my default browser on my work system. I wanted something other than Internet Explorer primarily for tabbed browsing purposes, but secondarily to help reduce the amount of memory that IE takes up.
My first switch was to Maxthon, which internally runs IE but handles tabbed browsing and mouse gestures. I now pretty much expect my browser to handle mouse gestures and get frustrated if holding the right mouse button and moving down and then right doesn't close the tab.
Maxthon was really very good, except that there was no plugin available to handle Google Bookmarks, which solved another thorny issue for me.
Firefox sounded like a great solution except for the number of sites I use (primarily inside Intel) that require ActiveX controls and other IE specifics. I bumped across the solution to this recently when running into an extension called IE Tab. This allows you to load IE within Firefox, mostly taking care of those issues. It hasn't worked with Test Director (and neither does their plugin from 1999), so I'm not entirely off IE, but I'm nearly there.
Oh, and NTLM authentication for the tons of Intel sites that need it? Yeah, it's got that...