via: http://www.webupd8.org/2011/05/install-firefox-nightly-from-ubuntu-ppa.html
Add the Mozilla Daily PPA (available for Ubuntu 11.04, 10.10 and 10.04) and install Firefox Nightly using the commands below:
$ [sudo] add-apt-repository ppa:ubuntu-mozilla-daily/ppa
$ [sudo] apt-get update
$ [sudo] apt-get install firefox-trunk
Since this is a daily builds PPA, it's nowhere near stable so use it at your own risk!
Once installed, you should find a new shortcut in your menu / Dash called "Nightly Web Browser" (not Firefox!).