Default repository on Ubuntu 10.04 prevent us to have version of Firefox above 3.6.x, but with this simple workaround, you can get newest version (7.0.1) for Firefox and Thunderbird, also.

What you have to do is adding this line into /etc/apt/sources.list:

#Firefox-Stable
deb http://ppa.launchpad.net/mozillateam/firefox-stable/ubuntu hardy main
deb-src http://ppa.launchpad.net/mozillateam/firefox-stable/ubuntu hardy main

Do the regular:

$ sudo apt-get update
$ sudo apt-get install firefox

And it's done! :)