For context, I recall that for years and years, Firefox was the highest ranked mobile browser. Mozilla invested a lot in mobile, Firefox devs had to rewrite the Android linker, invent new ways of starting binaries on Android, etc. just to make Firefox work (all of which were later used by Chrome for Android).
It still didn't make a dent in mobile browser shares.
Sure, Mozilla could have invested even more in Firefox mobile, but at some point, this would have come at the expense of Firefox desktop, which was the source of ~100% of the funding.
What Firefox was doing 4 months after Android 1.0 GA'd would indeed been unlikely to have made a dent in mobile share compared to what effort was going on once Android had a billion users. Why put all of that effort in before something is even used to just then let it rot for years anyways? In the end, they ended up spending the resources to refresh it in 2019 anyways - by which time billions had already decided Firefox was just a battery hog and slow on mobile.
It's a sad story because Firefox was so good on mobile when nobody had a chance to use it then it was crap when they did. On desktop Firefox is still the #1 non-bundled browser, things went so poorly on mobile they can't even come close to that today. In a parallel universe timings were inverted and Firefox may have even had more users on mobile than it does desktop today.