This has been brought up before (not here, just in general). The short answer is they heavily customized the analytics so it's not as 'bad' out of the box. You can read more about it below.
You can probably ask them directly if you'd want more of an answer. They don't seem to be trying to hide anything.
https://bugzilla.mozilla.org/show_bug.cgi?id=1122305#c8
Edit: also, as far as I know, Firefox actively should be blocking Google Analytics, unless they changed it (which is possible). About four years ago, Firefox started blocking Google Analytics by default.