When you try to install Firefox from a clean Windows install, Microsoft gets desperate. The default search engine is Bing so if you search for "download Firefox" you get a large ad-like panel above the results telling you how great Edge is and that you really don't need to change browsers. Then when you finally install Firefox, it'll put ads in your start menu to "try Edge", I've even seen it add a little popup when I tried to open Firefox though I don't know if they still do that.

Microsoft disabled the API that let's you change the default browser because Chrome and Firefox kept doing that automatically when you clicked a button. Mozilla reverse engineered the new method of setting the browser but Microsoft then cried foul because you're not supposed to be able to set the default browser from within your preferred browser unless you're Edge.

And then with every major update you risk your browser preferences being reset because of "bugs". I've had Google set as my default search engine in Edge (to avoid the desperate ads from MS) but that got reset one update. The default browser got reset twice. Microsoft assures us that these were bugs and mistakes and that they didn't intend to change user settings, though.

I have a hard time believing any of it ever since the day I got an ad for their browser in my start menu.