According to Tipping Point's Zero Day Initiative, the vulnerability, which it rates as critical, was reported within the first five hours of Firefox 3's release.
"Once the vulnerability was verified in TippingPoint's DVLabs and acquired from the researcher, the vulnerability was promptly reported to the Mozilla security team," said a representative.
Although the Zero Day Initiative team does not offer specifics until the vendor has a chance to patch it, the blog post did say this vulnerability, which also affects Firefox 2, requires user interaction and could result in an attacker executing arbitrary code.
Mozilla is reported to be working on a fix.
The Zero Day Initiative has been criticized in the past for paying researchers who find vulnerabilities.