Never use firefox, mostly chrome, but IE is required for Deltek timekeeping with this company. Last company, Deltek worked on Chrome for everything but starting a new timesheet. Odd, Deltek. But we have all three on all our machines.
Chrome. Firefox suffers from horrible bloat. I like that each tab in Chrome has its own process and memory space, thus if something in one tab crashes it doesn't kill your browser. Also I like that Chrome integrates Flash and somehow often has updates pushed out when there are Flash vulnerabilities (like every week) before they are generally downloadable as a separate component for other modular browsers.
Opera is a nice "boutique" browser to have as a standby, used to be much faster, but still reasonable. I avoid IE like the plague.