I have written a tutorial about how to install Firefox 4.0 Beta 3 pre in Ubuntu few days ago.I think here is a permanent way to upgrade or install the most recent Firefox release or beta release,and it is Ubuntuzilla.
Ubuntuzilla is a python script for Ubuntu user to easy-download and install the latest stable Mozilla Firefox release or a special release(beta release included).Absolutely,Ubuntuzilla is not only a Firefox installer.It is also hosting the Mozilla builds of the latest official releases of Thunderbird, and Seamonkey.

Download Ubuntuzilla:

Go to following page and choose to download an appropriate deb package at the bottom,then double-click on the package to install it.

http://sourceforge.net/projects/ubuntuzilla/files/

Use Ubuntuzilla to upgrade Firefox to the latest release or install a special release:

Once Ubuntuzilla installed in your system,open up a terminal window and execute this command to start this script:

ubuntuzilla.py -a install -p firefox

It outputs the most recent release of Firefox and asks if you would like to install this latest release or a different release (In firefox window,go to Help->About Mozilla Firefox to check the current used version).

The most recent release of Firefox is detected to be 3.6.8.

Please make sure this is correct before proceeding. (You can confirm by going to http://www.mozilla.org/)
If no version number shows, if the version shown is not the latest, or if you would like to install a different release, press 'n', and you'll be given the option to enter the version manually. Otherwise, press 'y', and proceed with installation. [y/n]?
Please enter 'y' or 'n': y

1.Input “y” to install the latest release and it will list all available languages for Firefox.Input a number to choose your language and it will start the installation immediately.

2.Input “n” to install a different release and it will prompt you to enter the version of Firefox you wish to install (Note that beta and release candidate versions are now allowed, but you use pre-release software at your own risk!).