The only way Firefox is going to gain mass adoption is getting shipped as the default browser on new computers. At that point in time it will not matter, you will see your firefox click through rates skyrocket. Why? Because people are just using it because they don't know there is an alternative out there. Plus if it works, why change it? So the only people that move to Firefox are either techno-savvy or have friends/family/geeks that get them to use it. Those are the ones who click on your ads (the second group).