See http://blog.sidstamm.com/2012/02/malware-and-phishing-protec... and http://bugzil.la/368255

Google's Safe Browsing API (malware and phishing protection) requires a cookie. But Firefox puts that cookie in a separate bucket than the one used for regular requests. If you have cookies enabled, you'll actually end up with two separate google.com "PREF" cookies - one for regular HTTP requests, and one just for for updating the Safe Browsing lists. Disabling Safe Browsing will prevent that cookie from every being sent, and should allow you to delete it (EDIT: modulo the cookie manager bug below).

UPDATE: It looks like http://bugzil.la/1026538 is why the cookie keeps reappearing.