I think part of this discussion needs to be that Firefox, while open source, doesn't provide footing for the community to build browser forks the way Chromium does. This sort of open usage, ironically, feels very fitting to the spirit of Firefox, but they're not even close to Chrom(e|ium).
I've been building a browser full time the past year+ (synth.app) and FF core wasn't really even an option. They've done very little to make the engine usable outside of Firefox.
Now I'm sure there are a variety of savory and unsavory things that led here, but building that stuff out seems like a good way to grow the ecosystem. At least they would've had a chance at capturing the Brave/Opera/Edge/etc market and those seats at the committees.
This isn't to say that I'm not worried about these standards merging like this. I've spent many hours working out weird chromium only behaviors that websites rely on these days to do things (and egregiously so if they detect an agent remotely resembling chrome).
I wonder how difficult would be an open source community project whose explicit aim would be to take the Firefox code base and make it more modular and embeddable, like Chromium? Would it be too much work for a community effort?
Electron, but based on Firefox instead of chrome.
Not the most appealing idea, but an interesting one. Might help move more people to essentially using Firefox.