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.

Google? :-)

By the way, it occurs to me that Microsoft is now doing better what Google was doing with the Chrome Frame for IE extension a few years ago.

13 years ago, I asked myself, why Microsoft doesn't just embed WebKit instead of keeping using their annoying web engine. It turns out they are now doing a variant of that now, something that seemed impossible back then, and (amazingly) I am not too happy with this... I guess I did not predict the collapse of my favorite browser's market share at the time, and the dominance of Chrome, which did not exist. Google seemed good anyway.

Google building a web browser was strange to me back then: why would Google do that when we already have Firefox, an excellent browser? But now, it is obvious. And Microsoft does not care that much apparently. They don't care about having an edge over this area anymore.

There were plenty of technical reasons at the time, remember Firefox/Mozilla devs were strongly against the multiple processes model that Chrome devs really wanted. Chrome has redefined what it means to build a secure browser and I'm happy this is getting adopted now. Ofc, in the meantime Chrome isn't just a better technical alternative to existing browsers anymore, it's a strategic asset with all that entails.