Firefox on Android is becoming ready for prime time just now, until recently it lacked basic functionality that most competitors have, e.g. unless you opted into beta, you still can't organise your bookmarks in folders [1]. So it is understandable that it still has a tiny user base, but at the same time now that critical features have been added I expect it to grow. We should also consider that soon, when version 58 will be out, there should be a significant performance improvement which will help adoption [2].

[1] https://support.mozilla.org/en-US/questions/1160470

[2] https://news.ycombinator.com/item?id=16047689