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).