Post date: 22-Mar-2011 22:21:35
sudo add-apt-repository ppa:mozillateam/firefox-stable && sudo apt-get update
Close all instances of Firefox. Next time you launch Firefox you will be greeted by 4.0. Optionally go to: http://www.webkit.org/perf/sunspider-0.9.1/sunspider-0.9.1/driver.html and enjoy the fireworks!
In case you are experiencing flickering and/or artifacts in Flash on 64bit Ubuntu you may want to look at this HOWTO.