Unless you're also okay with Google having root (!) on your phone, it also practically means there's no official package managed source for the binaries. Yes, I can probably grab an apk from some Mozilla domain somewhere (if I can even find it! all their links browseable from mozilla.org simply go to Google Play. The irony of needing to use Google to avoid Google), or I can maybe add an unofficial third-party repo to F-Droid (dangerous! bad!), but if I care at all about sensible concepts like security and regular updates there's no practical way for me to manage it. Even if Mozilla hosted their own official F-Droid repo, that would be better than nothing - but they do not.

Besides, F-Droid is a canary - if you can't get your app into F-Droid, in any capacity, then it's not free software. If you won't do what it takes to get it into F-Droid, you've also staked your colors in the ground - that you don't care about that value system, or about users who have that value system. The message is clear: "Digital serfs only - digital yeomans need not apply".