To install the latest Firefox 3.5 (final) in Ubuntu

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 247510BE

sudo apt-get update && sudo apt-get install firefox-3.5

sudo apt-get update && sudo apt-get upgrade

Update:

And finally, install Firefox 3.5 final:If you already have a version of Firefox 3.5 installed from a repo:-you may want to try out Switftox which is an improved build of Firefox which also reached version 3.5 (actually it's updated just days after Firefox is) and runs a lot faster and it even uses your Firefox profile so you'll still have all your addons and customizations.