I use FireFox (3.0.4) as my default, but I also have the following installed:
Opera (9.51 & 9.63)
Safari (3.1.2)
Chrome (1.0.154.36)
IE7 (5730.13)
I keep them all around to test my HTML, CSS & Javascript functions before launching web pages. I also have UserAgentSwitcher for FireFox, but I like to be thorough.
Actually, as I'm typing this, I'm getting a notification of FireFox 3.0.5.. time to update.