Well, actually, they "support" non-GMS Android devices in the sense that no Google services are required to run Firefox on Android. But you have to get the APK from unofficial sites like APKpure or APKmirror (trusting you're downloading a non-tampered file) or finding the appropriate build among all the releases in Github.
Perhaps the complaint is that they are not releasing to competing stores like Amazon? I would find it hard to believe that legit Firefox builds are not already on Fdroid
Right now only Fennec is on F-Droid (which has some changes that give you more user freedom than the default browser build…) but Firefox is nowhere to be found. Their build process is incompatible with the official repo but that doesn't mean they can't set up their own repo instead.
Not every Android device has access to Google Play, yet Mozilla still doesn't provide an APK download for Firefox from its website to make sure those users can have their browser too, despite all of this complaining.