Now that, that's clear. My answer is I don't know really. They used to maintain a repo back when the stupe Iceweasel thing was going on. A theory, maybe Mozilla still maintains bad feelings towards Debian for that Iceweasel fiasco ? Deciding to rebrand and fork the premiere open source web browser due to something like copyrighted naming/brand and imagery was pretty petty and imo foolish on the part of whoever decided to do so. It hurt both Debian and Mozilla.
Or perhaps it's simply the fact that no qualified or trusted maintainer(s) have stepped up and elected the responsibility of maintaining a Firefox repository ? Could definitely be something else too of course. Truly I don't care, have run Firefox directly from Mozilla for the majority of the time I've been using gnu/Linux and it works great. I get automatic updates as they come out and have no complaints. Am glad Debian opted to abandon Iceweasel and begin supporting Mozilla/Firefox again as it should be, shrugs.

PS, Too much coffee !!! Definitely think Mozilla should setup and start maintaining an official Firefox repo for Debian though. With Chrome(Google Inc)pretty much dominating the world in the browser wars, they need all the help and users they can get and keep.