Yes, in fact that you can download the source package, and uninstall the firefox 3.6 in Ubuntu and then install the 4.0 version, but how about when you want to own 3.6 and 4.0 at the same time?
Here is a way to do it.
Add the source.
sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa
Update the sources.
sudo apt-get update
Install firefox 4.0!
sudo apt-get install firefox-4.0
After that, you will find the firfox 4.0 is in Internet of the menu! Yes, the name of it is "Minefield 4.0 Web Browser".