> So, they're running Android and iOS on your
> computer to run the same binaries as those
No. "They" are allowing you to connect your Android or iOS device to your computer (likely via USB), then debugging the on-device browser using the Firefox debugger running on your computer. That way you're debugging the thing you actually want to debug, but using the same developer tools you're using for your other debugging, and which therefore you're already familiar with.