I'm pretty sure browser share is done by useragent, which will report Firefox if you're using Firefox. Unless of course you change your useragent but the number of people doing that will be vanishingly small.
To be fair, one of the very few add-ons available for Firefox on Android is an add-on that does exactly this on Google properties so Google won't degrade your experience for using FireFox.
So maybe it is more popular than we think. Very hard to know for sure.
