It was a recent change to the VRT ruleset from Sourcefire, please see here. It's in the policy.rules file (as I'm sure you know) and seems to be generated when an attempt is made to exploit a vulnerability in Firefox (a buffer overflow type attack).
Strangely the CVE link indicates that the vulnerability is from 2004 with a latest revision in 2008, which I suspect means the rule was always there but this is just part of the ir current re-working of Snort rules and therr categorisation.
This link here explains the vulnerability and exploit in much more detail.