Doubting things and skepticism are great. The 100m number is off common device profiling trackers (statcounter etc) but the number is also mentioned in several other sources.

You have not heard of it because it primarily targets feature phones and primarily in emerging markets. If you are not in the market for a feature phone in India or building software for one it makes total sense you're blissfully unaware.

(And as the other comment mentioned, after a very successful run as a FirefoxOS fork - Android caught up )