I’m one of the millions of people who downloaded Firefox 3.0 some days ago. Here is my certificate!
Unfortunately TippingPoint ZDI notified Mozilla of a vulnerability in Firefox that impacts versions 2.x and 3.0 has mention here. This issue is currently under investigation. To protect Firefox users, the details of the issue will remain closed until a patch is made available. There is no public exploit, the details are private, and so the current risk to users is minimal.
We look forward for a patch!