I have just installed the new firefox 4 on ubuntu and here is a quick guide to do so.

Open a new terminal window and copy-paste the following:}

sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa
sudo apt-get update  (this might take some time)
sudo apt-get install firefox-4.0

And you are ready to use firefox 4.

In my computer, I had to update manually some shortcuts, but this is not needed normally.
Bonus: You can test your new browser’s performance with Acid3. It scores really high, but to be honest the main reason I keep using firefox is the plugins.