This tutorial is going to explain how to install firefox 4 in ubuntu using PPA
You can check here what is new in firefox 4 from here
Install firefox 4 using GUI
Go to Applications > Ubuntu Software Center from the top panel go to Edit > Software Sources and click the ‘Other Software’ tab.Press ‘Add’ button and then paste the following line
After adding the PPA you will be prompted to update your sources.
Once the update is done you can head to System > Administration > Update Manager to perform an upgrade.
Install firefox 4 using terminal
Go to Applications > Terminal in the terminal just enter the following commands
sudo add-apt-repository ppa:mozillateam/firefox-stable
sudo apt-get update
sudo apt-get upgrade