s/you have a responsibility to/it would be most effective if you were to/, perhaps.
Otherwise, I agree with everything you said.
(And hey, I work for Mozilla, though not on UX stuff. When I saw the original description of poor UX, I was planning on checking it myself and possibly filing a bug if I agreed. But once the thread got into "abominations", I lost interest and subconsciously recategorized the complaint as coming from the subset of users who complain loudly and whose opinions I generally find too unrepresentative of more than a small niche. Which could be right or wrong in this case, but I have found it to be a very useful heuristic to consider the constructiveness of criticism as a signal of how useful it would be to examine further.)
BUT... i've read the whole thread, and the other too, and the only 'signal' between all the 'noise' of annoyed users is more or less: take it or leave it (because we say so)
This is arrogant. And politeness and politically correct speech under all circumstances leads to nothing but mediocrity, while the groupthinking celebrates the emperors new clothes, over and over again. I think sometimes it is necessary to bang the fist hard on the table, to recalibrate the signal processing. If this is too stressful for all the special snowflakes to bear, they maybe should stay in their safe spaces and don't babble about niche minorities. Or maybe smoke less weed?
And they released it for the same reason Slack released their input box update https://news.ycombinator.com/item?id=21589647. Some manager wanted to leave a mark and nobody was there to oppose them.