I upgraded my browser yesterday to Firefox 4, and I have to say that in general I'm quite pleased with it. New features, fast, etc.
As with the Firefox 3.5 upgrade a couple of years ago, Mozilla has a cool page to monitor the downloads worldwide, and once again, it is powered under the covers by my company, SQLstream.
In case you feel like learning how we do our part of the magic, one of my coworkers has written a very good explanation of how it works. Yay!