Comment by shadowgovt

You're basically looking at testing being done on Chrome (because it is Google's, and because of its large market share), Safari (because it runs on a large percentage of completely exclusive platforms where the customer can't switch, and because of its large market share), and Edge (because there are still many corporations that do "Nobody ever got fired for choosing Microsoft" and lock down browser options to just Microsoft's offering).

At this point, Firefox is very much an also-ran on two axes: market share is tiny and nobody forces it on their captive audiences. We may as well ask why Google isn't optimizing testing on Opera, or Samsung Internet.

(There is also the issue of under-the-hood engine. Since so many browsers have converged on a few core and JS stacks, testing on one exemplar of that stack has a tendency to suss out bugs in the other stacks. Firefox still being its own special snowflake in terms of JS engine and core means it has more opportunities to be different, for good or for ill. So there's a force-multiplier testing the other browsers that one lacks testing Firefox).