For all you folks who have been enjoying ubuntuzilla, well, this is not an official announcement, but it seems this PPA is now unmaintained. Hope I'm wrong. It was very nice while it lasted.
I found an official ubuntu build of the latest mozilla FF at:
https://launchpad.net/~ubuntu-mozilla-security/+archive/ppa/
To find your O/S release (e.g., 8.04LTS) click the green "Technical details about this PPA" link and select your O/S from the dropdown.
Follow the rest of the instructions to add the PPA to your sources.
Print this out: Exit firefox
sudo apt-get remove firefox-mozilla-build
(or follow instructions if you installed from the python script.)
Delete or comment from /etc/apt/sources.list: deb http://switch.dl.sourceforge.net/project/ubuntuzilla/mozilla/apt all main
sudo apt-get update
sudo apt-get install firefox firefox-branding
You should be at 3.6.15 or later.
I'm not sure what happens on the jump to FF4.
Good luck!