Phoenix Firebird Firefox is my browser of choice these days, but I keep IE around, too. Like it or not, you have to test with it. The vast majority of people use IE, so any web apps you write have to work under it. It's no good pointing out that your lovely code is failing 'cos of an IE bug; you just have to work around it.

(Also, we have an internal timesheet system which isn't too chipper under Firefox. Nothing to do with me, I assure you.)

I use IE View to open IE from Firefox; you just right-click on a page or link in Firefox, and the context menu includes a handy little 'open in IE' option. It seems that doing the same thing the other way isn't too tricky either: Extending the Internet Explorer context menu.