There’s a beta version that works with Firefox 3. The built-in updater doesn’t install beta versions, I don’t think, so it says there’s no update. But you can install the beta version manually and it works.
The firebug site was down for days and days recently. Very bad timing for the release of FF3. I really needed a version at one point and had to get someone to zip it up and send me a copy from their local machine.