it is a referral program that pays for each download of firefox browser https://www.google.com/adsense/static/en_US/Referrals.html
As said by Sultanofseo,.. it is a referrel ad by google. You will find that in 'get ads' page in adsense.
i have been running firefox refs for the 3 days i have had adsense and so far made $10.60 10 were $1 1 was $0.5 1 was $0.1 so not bad but depends on traffic
about 98% of my FF referral conversions pay at the full $1 - this is because 98% of my traffic is U.S. based. Google has some good reading about earning with Firefox conversions HERE and HERE ..
I had firefox referrals on about 420 pages in one of my sites. In 4 months, I earned $0.20 I have removed them all.
Yes, Google worked out whatever the problem was with tracking conversions about 1 week after it started.. My conversion earnings are back to normal
thats great to hear. however, its wierd that google had issue tracking conversion. that suggests you must have lost a lot of money
It depends of your site as to how successful firefox referrals will be but for the majority of us, our traffic wont be interested enough to click so we use normal ads which are targeted.