First of all usage of Firefox is mostly desktop. On iOS you don't even have a real choice. Just looking at desktop doubles the share. Then you have different usage in different countries. Looking at desktop in Europe you get above 10%. Last I'm never sure you skewed the Firefox stats are. Often 3rd party trackers are blocked and I have yet to find a comparison of client side vs. server side tracking. Not saying the Firefox numbers are wrong, just that they aren't certain.