"just for Windows" probably still makes it one of the biggest repos in the world. I wouldn't be surprised at all if it's actually the biggest.

Correct. The project I work on at MS is 3 repos all by itself.

No, it doesn't. Firefox is a monorepo (of course, it's just one product). All their other products (bugzilla, rust, all the web properties) have different repos. Check out their other repos: https://github.com/mozilla

Also, JaneStreet