I guess it is just not a popular feature on mobile. Most smartphone users don’t even know what a browser is, so ensuring addon support seems to be very low priority for Mozilla. Even a switch hidden somewhere in the about:config page in Firefix Nightly would be enough. These extension collections are the most user-hostile way of extension support possible. I just switched to Kiwi because of that mainly.
I think there are some Firefox forks with the entire extension store enabled, but I have not looked into this.