Despite it's code name, I'm quite enjoying the Firefox 4.0 beta. I was forced into an upgrade due to some weirdness with libgif and animation, but so far I don't regret it. Seems genuinely faster, especially on sites that rely heavily on Javascript/AJAX. Only down side is few extensions don't work with it yet, but AdBlock Plus, NoScript, and FoxyProxy do.
Despite seeming to back away from 64bit support, Adobe has backtracked a bit and released new 64bit betas of Flash.