If you want to use firefox 3.5 before it's officially in the repositories, you can just go to http://www.mozilla.com/en-US/firefox/upgrade.html , download firefox, untar the package, and run the firefox bin inside. Just make sure that when it asks you to make firefox 3.5 your default browser you answer "no"; you should wait until you get the official package before making it default.
Anyway, cut the ubuntu guys some slack. They have to do a comprehensive quality check on Firefox 3.5 before they allow it into the official repository since it's such a crucial piece of software. Give them a couple more days or even a week, it'll get there.
I wish meatspace had a search function.