What evidence, short of Mozilla going ahead with it and Firefox market share tanking over the course of a few months, would be sufficient to persuade you that this is a bad move?
(I do imagine that there should at least be ample precedent out there for the "management of ~10% market share company decides to sacrifice strong point of its own product to pursue the strong point of the market leader instead" pattern. I can think of several failed examples, but no successful ones - are there any? If not, I believe that should count as evidence that this is a common bias in company judgement.)
Regarding b): at least the author of DownThemAll!, which has ~1.2m users per [1] (which makes it the non-adblocker plugin with the highest number of users I can find), has come out saying they won't [2].
...and regarding a), I don't think that's all there is - FOSS projects at this point have a quite long track record of changes that were championed by the marketing crowd over the skepticism and outright opposition of the "fanatic techies" of the kind found here, such as KDE's Plasma Mobile and Mozilla's own phone OS, which turned out dead in the water exactly as the "unrepresentative" crowd expected.
(I do imagine that there should at least be ample precedent out there for the "management of ~10% market share company decides to sacrifice strong point of its own product to pursue the strong point of the market leader instead" pattern. I can think of several failed examples, but no successful ones - are there any? If not, I believe that should count as evidence that this is a common bias in company judgement.)