I feel like it’s necessary to mention that I’m just speculating, and don’t have any affiliation with Adobe, thus I can’t say for certain that I know why they choose to not support Firefox.

I’ve been in the position before though where I’ve chosen not to support non Blink/V8 browsers for the reasons listed above.

The fragmented nature of the web platform makes it a pain to develop for, in a way you don’t necessarily experience with “real” languages.

I have been, and honestly still am, of the opinion that Mozilla should just forego their engine and move to Chromium. Not because one is better than the other - if anything I think Mozilla’s implementations are, as they tend to be more “by the book” - but in unifying the web platform it’d be easier to develop for, and it would bring the added bonus of Google not having as big a monopoly on what goes on in Chromium.

Microsoft moving to Chromium was big in that sense, so I’d love to see an established FOSS vendor like Mozilla exert their influence on the project.