As an avid Firefox user, privacy tinfoil hat and all-round Stallman-esque interjector, I wouldn't mind if Microsoft contributed to Gecko, Quantum and SpiderMonkey - provided it didn't interfere in any way with my abilities to do for example ad- and tracker blocking.

As far as I understand it, the Chromium codebase is deeply intertwined with the Google ecosystem in a way that makes it hard to guarantee privacy the way Firefox does.