Though i must say,i don't understand one thing.I am using this particular ppa for sometime now to try firefox-4.0;and i expected the beta version to appear beside firefox-4.0 and not beside firefox(3.6.17).
A little bit confused due to this;anyway will upgrade tonight.
If you do not have this ppa installed already,type this in terminal
sudo add-apt-repository ppa:mozillateam/firefox-nextsudo apt-get update
sudo apt-get install firefox