There is no magic fairy dust protecting Macs - Dai Zovi, security researcher and co-author of The Mac Hacker's Handbook.
Its been around since June 2006, and some said it was fixed in Firefox 1.5.0.7, but it's back...http://www.securityfocus.com/bid/19488
Further information:http://nvd.nist.gov/nvd.cfm?cvename=CVE-2006-4253