A side remark in this recent add-ons blogpost: Introducing a scalable add-ons blocklist - Mozilla Add-ons Community Blog may shed some light on the factors involved with allowing the installation of arbitrary extensions on Android:
We are currently evaluating additional optimizations in order to further minimize the size of the blocklist for use on Fenix, the next major release of Firefox for Android.
This may of course also not have been a factor in the decision.