Firefox has more than a 4% market share[0][1]. One of the main reasons I've heard, is that it's free, and that the Mozilla Foundation is a non-profit.

I also just found this bit, that explains how some browsers might be over or under estimated, because of how their internals work[2]. So we really don't know what the market shares really are.

[0]http://gs.statcounter.com/#browser-ww-yearly-2008-2016

[1]https://www.w3counter.com/globalstats.php

[2]https://en.wikipedia.org/wiki/Usage_share_of_web_browsers#Ac...