I guess the folks at Mozilla decided to push out a release candidate ahead of the anticipated IE9 release at SXSW, as the Firefox 4 RC has been officially released. It appears to still support profiles, and I was able to load up the AdBlock Plus, Xmarks, and LastPass extensions that I always use. (Yes, I run FF 3.5, 3.6, 4.0, and Minefield on the same machine). Based upon the nightlies that I have been using, there are some definate UI changes from the 3.x series of Firefox. One big thing is that Firefox 4.x will still run on XP, unlike IE9.