Looks like the overload theory is correct:
The outpouring of interest and enthusiasm around Firefox 3 has been overwhelming (literally!). Our servers are currently feeling the burn and should be back to normal shortly. Download day will officially commence once the site goes live. The 24 hours period will be clocked from that moment. Thanks for your continued support.
Oh what fun!
Update: It’s now working, and I’m running the final release of Firefox 3.0 Mac version