Chromium - chrome, edge, brave, V8, V8 isolates, node.js, electron, etc. Firefox - Firefox, Torbrowser, spidermonkey. Chromium project became the cpython of JavaScript and web browsers, because you could package it's components and embed them in your own applications. Firefox does not have competitive equivalents or better features. Therefore it loses users. This was a big topic years ago when FF started removing XUL…
XULRunner was Electron before it was cool. Though they werent prevalent there was a small but promising variety of applications based on XULRunner [0]
Then of course Mozilla administration axed it in their infinite wisdom.
[0] https://en.m.wikipedia.org/wiki/Category:Software_that_uses_...