Firefox is not a product of the 800lb gorilla of targeted ad revenue. The Mozilla foundation could live without the ad revenue tied 3rd party cookies. They feel pressure from their users (people like us) who want to not be tracked everywhere, and ending the use of 3rd party cookies is one technical step.
At some point in the past year it seems the privacy concerns reached a critical mass. The Mozilla foundation is responding in ways to keep its browser share relevant.
