Check the cool new features of Firefox 3.5 on the Release Notes and start downloading people
Be prepared as most of your addons will NOT be available after you upgrade as they are not listed as compatible with Firefox 3.5 (for now). It would take few days to weeks to get the new one compatible with FF 3.5. Anyway, it's good to know that some addons are already working with FF 3.5, such as Greasemonkey, StumbleUpon, Firebug, FireGPG, ColorfulTabs, Better GMail 2, and AdBlockPlus.
After you upgrade, check the About Mozilla Firefox and you will get something like this: