I do not think that this was that confusing. People [1] looked around at the beginning of the 2010's and saw

1) Mobile usage was growing exponentially and desktop was... not [2].

2) Every mobile OS shipped their own browser by default, or even went so far as to prevent other browsers from being used at all (iOS) [3].

3) Because Android and iOS both had non-trivial marketshare, neither could be called a "monopoly" so there was no way to use anti-trust law to get Firefox on devices as was done with Windows (not that this would have been a compelling strategy even if it were possible).

People took that set of facts and concluded Mozilla needed its own mobile OS in order to stay relevant.

What they underestimated was the amount of investment needed to make such an OS and get it on devices and the amount of time it would have to exist in a state of not being very good before it could compete with the established players (who were not standing still... people forget how bad Android was in the beginning). But if you look at the actual world we ended up in, with no mobile OS from Mozilla and a total Firefox marketshare that is less than desktop Safari's, it is hard to say that initial conclusion was incorrect.

[1] Full disclosure: I was a Mozilla employee at the time, though not involved in any of these decisions.

[2] I would say "desktop was shrinking", but to everyone's surprise it actually remained fairly steady in absolute numbers, although it did become a smaller slice of a much larger pie. In 2010 everyone expected it to shrink, though.

[3] Mozilla did ship a re-skin around mobile Safari to try to get some brand presence, but was still at the mercy of what web standards Safari chose to implement, and you could hardly call it a first-class experience. Eventually iOS loosened their rules, but no one could have predicted that back then.