https://gs.statcounter.com/browser-market-share/desktop/worl...
Edge is available on Windows, Linux and macOS, so it would probably do. But that would allow one of Google's biggest competitors to drop an under-performing product and lobby for antitrust against Google. Unlikely to happen, but a risk Google might not want to take.
https://gs.statcounter.com/os-market-share/desktop/worldwide
There's such a marginal difference between the quality of the two browsers, and Chrome is held back in what it can be by the necessity of furthering Google's commercial interests. The only limit Firefox has had is that they can't abuse the trust of their users. Firefox had to voluntarily (and often aggressively) inflict a huge amount of reputational and functional damage on itself to reduce its market share to the place that it has.
edit: it's important to say that they didn't really backslide technically; it's user-hostile (management) decisions that have hurt the browser, not anything to do with the skill of Firefox developers.
Firefox is the only "credible" competitor, although Firefox's only profitable customer is Google itself.
It is questionable, and being declared a search engine monopoly would be far worse for Google than Chrome being a browser monopoly. They only make Chrome to push their search/ad network.