Firefox (by that I hope you mean Mozilla Project) cares about diversity and choice. Our Mozilla Manifesto [0] describes our vision of the balance between corporate and civic value of the Internet.
I speak for myself - I believe that decrease in diversity of implementations of the Web standard and drift toward a monopoly is harmful to our vision of the Internet and ultimately will hurt the Open Web.
I mean, in all honesty, EdgeHTML wasn't ever really an engine that had much hope. IE has always been loathed, Edge just continued that trend. Their marketshare wasn't great, and tying browser updates to the OS as they did with IE was irresponsible for how browsers are shipped these days.
If Edge had 35% marketshare I'd be more concerned, but nobody was using it, and web developers hated trying to make their sites compatible.
I speak for myself - I believe that decrease in diversity of implementations of the Web standard and drift toward a monopoly is harmful to our vision of the Internet and ultimately will hurt the Open Web.
[0] https://www.mozilla.org/en-US/about/manifesto/