Installed: Firefox 4.0 Beta 6, Minefield 4.0 Beta 8 Pre-release (64bit), IE 9 beta, IE 9 Beta (64bit), Chrome Developer Edition (8.0.552.0 dev) and Opera 10.XX (beta).
I use Minefield daily, IE every once in a while when something won't load correctly in Minefield (or Fx) and Chrome updates itself from the dev channel.
I stick with Fx b/c I can move the profile where I want to, create it where I want to and if I have to uninstall / reinstall it and / or format my machine, I just install, run profile manager, point it to my existing profile, and
poof! all of my stuff is right there, extensions settings, the whole nine yards.
Right now I am using Minefield extensively b/c of the (finally!) 64bit Flash build and the fact that my minefield is 64bit as well. In fact, typing in here from Minefield.
Oh, and I have the latest Fennec 4.0b2 installed on my DROID as well. As soon as the next nightly comes out I can enable sync and then all my stuff on my computer is on my phone. BMs, tabs, history, all that jazz