The main reason many people abandoned Firefox was because everything about Chrome was faster and more responsive. Render engine performance matters more than implementing APIs that one or two websites may ever use for anything other than browser fingerprinting. Servo was abandoned by Mozilla but the improvements to the browser engine that did make it into Firefox have sped up the browser significantly. They were the reason for a whole bunch of "check out Firefox it's fast again" posts all across the internet.

I care more about my browser being fast than I do about it supporting WebMIDI or WebSerial and I think most users agree with that. Performance and efficiency are also the reason (as far as I can tell) that macOS users stick with Safari.