While I agree with you and the way you’ve stated it, it should be widely known that tiny groups of highly technical people can unlock huge understandings about how to improve.

Look at the speedrunning community for example: sometimes it’s not just a tiny group, but a single person that sees something that the devs did not, and that can lead to fixing “wtf” bugs for everyone else.

There is a chasm of difference between "We've identified something you have not given consideration to" as you describe (which I wholeheartedly support), and "We will insist we're right about the same points over and over again in every post about Firefox for all time" as HN behaves (which I do not, no matter what the topic). The former is a valuable and essential component of public issue reporting; the latter is not.

I see Firefox doing user hostile activity all the time, so I’m more inclined to think that are using telemetry in the slowly boiling frog analogy. The extension download data even without telemetry shows people want multi row tabs but they’re not putting it in by default. I am not sure how much the data is being used to improve user experience.

Which extension do you mean?

Multi row tab.