Firefox's much-lauded 3.0 release has been official for only a day, but already reports of security vulnerabilities have come out. DVLabs has reported that they actually probed and discovered a flaw within five hours of launch via their Zero Day Initiative program. However, the flaw itself might have been discoverable earlier - as it also affects older versions of Firefox including the 2.0.x branch.
Unfortunately for curious folks, DVLabs is not revealing much information on the flaw, but do note that they have reported it to Mozilla. They claim the fact it came so shortly after release is coincidental. Obviously flaws found in the beta series do not apply here, as the entire purpose of betas is to squash bugs.