As a frontend dev, every company I've worked for this decade has deliberately chosen to drop Firefox support. We've never gotten a single complaint about it from our users (we get plenty of other reports) and every analytics report showed decreasing usage.
The few developers I've met who still use Firefox eventually stop once we run into the first issue with it on our own projects (usually having to do with graphics rendering of some sort). We don't fix it for our users so we're not going to go out of our way to fix it for our own frontend devs. It's just not worth the time investment.
Firefox is totally dead aside from a few ideologues, who don't really constitute enough of the general population to matter. Webkit support (for ios browsers outside Europe) is much more important, and it has enough differences from Blink to be a time sink on its own, so all the extra dev time that might've otherwise gone to Firefox instead go to Webkit tweaks (with Chrome/Blink being the standard, both for users and devs).
Browser compatibility is a lie, even with polyfills and caniuse and browerslist. If you do enough full time frontend work you'll soon come across the various subtle bugs and incompatibilies. Browsers aren't necessarily more standard now, it's just that Chrome won in a way that even IE didn't.
I grew up in the IE era and did web dev then too. Yeah it's better now, but I think it's mostly because Google has such a monopoly that they are the real standards. The W3C is completely irrelevant now and WHATWG seems like basically a puppet government for the big corporations to dictate what they want. At the end of the day, Google decides what is going into Web, period. Apple controls half of mobile (apps) but Safari is always the odd duck. Firefox... nobody cares about anymore.
Kinda sad, as someone who loved early Phoenix, but honestly Mozilla has so thoroughly mismanaged Firefox over the years that I'm surprised it has any users left at all.