Installing the latest Firefox 3.6 release in Ubuntu is easy. You have to get the packages from Mozilla rather than Ubuntu, as Ubuntu is much more cautious with their release cycle, and with the rate they import packages into their system. This is a step you will only have to do once, and then you will have access to all of Mozilla's releases designed for ubuntu. Simply type the following commands to install Firefox 3.6:
sudo add-apt-repository ppa:ubuntu-mozilla-daily
sudo apt-get update
sudo apt-get remove firefox
sudo apt-get install firefox-3.6
The step to remove existing firefox installations may or may not be necessary for you. On the machine I tried it on, I had to remove "firefox" before the new required packages for firefox-3.6 would install properly. Don't worry, as installing/uninstalling software should never affect your user preferences.
What's awesome in Firefox 3.6?
One of the best things about Firefox 3.6 is their new concept of "personas". Rather than a full theme, personas are more similar to a desktop background. This means the persona is going to affect the color and images used in the browser, but not much else. The best thing about personas is that they can be previewed live in the browser simply by hovering over each of the themes, and apparently there are already 30,000 themes, most of which are pretty cool looking. Check out getpersonas.com to browse the entire catalog.
The second most interesting thing is that Mozilla has continued improving the speed of the browser overall, and apparently it's 20% faster than Firefox 3.5, which is great for a browser that most people love, but has been receiving a lot of flak for being slower than Chrome.