Not available in the Fedora repositories yet, so I had to install manually.
For reference, untar to /usr/local/lib/firefox-4.0, then copy the 'firefox' shell script from that folder to /usr/bin/firefox to replace the existing one. Torbutton doesn't comply, and neither does the English language pack!
It's much nicer than 3.6 on my netbook - much more screen space which makes a lot of difference when you've only got 640px to play with!