Fire fox has released the first update for Firefox 3.0 to 3.0.1 on July 16
Most will automatically get the updates if running version 3.0, to check click on the help tab in Firefox, and then about, if it shows 3.0.1 you have already received the update.
You can manually check for the update as well, by clicking help and then check for updates.
For what's new you can check it here
http://www.mozilla.com/en-US/firefox/3.0.1/releasenotes/
