Firefox 3 download day is June 17th, so if it's June 17th in your time zone now should you download it?
No! Ok, go ahead now if you can get in.
In what I can only describe as a monumental screw-up, the Mozilla foundation decided to start the "day" at 10am PDT (1pm EDT). So wait a few more hours folks.
Then go to the world record site. If it still has a button to pledge (as opposed to one to download) then the event hasn't started yet.
Update: Mozilla now says "Firefox 3 is currently only being offered as a full download from http://getfirefox.com/". The event has now started, but you may find that the download site is overloaded.
Microsoft did it right in 1996 when they hosted a "midnight madness" event. The first 10,000 people that downloaded IE3 got a T-shirt (I still have mine). I've read in the forums that some people stayed up late for Firefox 3 download parties, only to be disappointed that the download links were not active. Or worse, that they got one of the release candidate versions of FF3 and their download doesn't count towards the record.
I know... download parties, world records, it's all pretty silly. But who said computers had to be all work and no play?