Beside this, Firefox is not really welcomed (and a drop is planned eventually) on F-Droid because it's really complex to build, depends on some closed source libraries (Google Play Services, for example) and has lots of tracking in it (everything is at least opt-out as far as I can tell, but the guys behind F-Droid aren't really keen on any kind of tracking it seems).
It's possible to build it without those problems (see Fennec on F-Droid in fact), but it takes a lot of time for someone to create a recipe, and it's prone to break on every upgrade (and that's why the last version of Fennec on F-Droid is based on Firefox 40).
I don't know if/how many of those problems apply on Firefox OS (I guess at least the complex build environment applies), but that's likely why you'll not find it on F-Droid anytime soon. Unless someone decides to pour lots and lots of energy into this.
More info/sources:
https://f-droid.org/wiki/page/org.mozilla.firefox
https://f-droid.org/wiki/page/org.mozilla.fennec_fdroid
https://gitlab.com/fdroid/fdroiddata/blob/master/metadata/or...