It is very easy to install Firefox 5 Beta on Ubuntu via ppa thanks to WebUpd8 for the update, all you need to do is follow below instructions:
Open Terminal and type:
$ sudo add-apt-repository ppa:mozillateam/firefox-next
$ sudo apt-get update
$ sudo apt-get upgrade
$ sudo apt-get install firefox
(installation instead of the upgrade)
Done. Firefox 5 Beta 2 is installed and ready to roll.