Okay, I'm gonna pretend I don't know anything about downloading and installing a browser and just go the Microsoft Store and search for "Firefox". Yup, it's there. Now, I'll try for Google Chrome. How odd, it isn't available.
Surely the browser that you can't even get from Microsoft's app store would have less market share, right? Nope, according to the data, Chrome is still on top, despite the fact that you have to manually download and install it on Windows.
I doubt this occurred to Mozilla, but perhaps people just don't want to use Firefox? Crazy, I know.