The MX debs are just the Mozilla-provided prebuilt static binaries (hence our version's name) that we scrape from their ftp download archive, and then bundle into a deb with some extra configuration files, desktop file, and other goodies. I added the "Depends" for those based on what Mozilla had listed on their site for Linux system requirements at that time. I think they are built on as old a version of Centos as they can get away with.
Natively built-from-source binary debs, which is the "Debian way", get most of their dependencies automatically during the packaging as the finished binaries get queried about what they require, and those can vary quite a bit depending on what releases' library versions they get built against. It seems Mozilla's are getting built the Debian way, unlike our time-saving (and rustc-updating-avoiding method

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Debian has never stopped building their own debs for firefox and firefox-esr; perhaps the author's excitement is due to the fact that Ubuntu switched to only snap Firefox packages years ago.
This thread's title should be more about their new apt repo, IMO, since those include the debs by default.