If I remember correctly, Brenden Eich was a big proponent of Firefox OS, and it died shortly after they pushed him out.

Turns out people wanting a touchscreen want a much smarter OS. If Firefox OS was adaptive for non-touch devices - who knows?

Mozilla took the slowest HTML rendering engine there is and created a whole OS using it. Then they proceeded to run it on the worst mobile hardware there was at the time.

Then competent engineers forked it into KaiOS and it became a success.

Same engineers in notable cases, e.g., Fabrice Desre. Also same engine. This refutes your first sentence above.