On November 4th Google released an AdSense & Firefox Referral Program and yesterday they allowed the Firefox Referral Program to Go International. A relatively new WebmasterWorld thread named Firefox referrals - experience of others? shows that all those in this thread, have tried and all have removed the firefox banner ads from their sites. The none contextually relevant ads have a low CTR and conversion rate.
I guess these ads don't work too well. :)
Forum discussion at WebmasterWorld.