> if Firefox was just a small(ish) browser catering to power users. There's nothing wrong with that

Except that it couldn't work in today's world, as more and more web sites would stop working in Firefox. Firefox needs significant market share to survive with an independent browser engine. Having an independent engine is key to achieving Mozilla's mission: https://www.mozilla.org/en-US/mission/

You can build a browser focused on power users if you don't need to worry about the engine. Perhaps you want to take a look at Vivaldi?