Firefox 4 works well for me on both Windoze 7 & Mac OS also, no issues at all so far, & it's a lot faster than 3.(whatever the latest one was), which was getting tiresome.
I tried using Chrome as it was a lot faster than 3.x for browsing, it just had a lot of stuff missing like password managing (unless it was just me that couldn't find it, or it's an add on)
Not tried it on Vista, but have 4 XP pro machines at work so will see if it works as well on them.