Here are a couple of random notes about Firefox 4.
Linux distribution: To the best of my knowledge, PCLinuxOS was the first to distribute the final release of Firefox 4. That is based on the fact that when I got to work yesterday morning one of my colleagues who runs PCLinuxOS was already gloating about it having come in with the latest batch of updates. It was also updated on openSuSE 11.4 on one of my netbooks yesterday evening. I'm sure there are various other Linux distributions which are already updating to Firefox 4, or will be in the next few days. But keep in mind that some, such as Ubuntu (and thus most of the Ubuntu derivatives) won't include it until their next major release.
Worldwide downloads: If you haven't seen it already, there is a very cute web page that shows a World Map of Firefox Downloads, with a pinpoint flash of light each time a download is made. It's quite enlightening (no pun intended), on several levels. First, the frequency and distribution of the downloads; second, watching the areas which have very low download rates. Very interesting.
Download rate: As I write this, Firefox 4 has been downloaded well over 13 million times, in the first two days after it was released. I recall Microsoft making a big deal of the fact that Internet Explorer 9 was downloaded over 2 million times in the first day. Of course, the geniuses at Microsoft shot themselves squarely in the foot when they decided to snub Windows XP with Internet Destroyer 9, and make it for Vista and Win7 only. Firefox 4 works just fine on Windows XP... Maybe Microsoft should change the IE6 Countdown web page, and remove the "6" specification, so they can count down the death of Internet Explorer!
Personal Experience: I have installed Firefox 4 on the Windows XP system I (must) use at work, and as I mentioned above my openSuSE 11.4 and PCLinuxOS 2010.12 systems have already updated to it. I'm quite impressed with it so far. It is definitely faster, it has a lot of nice new features and improvements, but it is not so radically redesigned that I didn't recognize it or couldn't figure out how to use it when I first started it. There was one small change that I didn't care for, which seems to be a common objection based on the fact that it is the single most frequently mentioned "tip" I have seen in announcements and reviews. The window "tabs" have been moved above the address bar, rather than directly on top of the windows. If you don't like that, simply go to the menu bar, View/Toolbars/Tabs on Top.
Summary: This is an excellent new release, and it appears to be getting the attention and acceptance that it deserves. Good stuff.