So this is technically returning an old feature. Prior to the admittedly much, much needed revamp of the app, basically any desktop addon would “just work” on mobile. After the update, only select addons approved by mozilla “just work”, for all other addons you have to use the dev/nightly build of the app and then enable a config flag. This new update is essentially a return to the old system for addons but without sacrificing the performance benefits the revamp brought.

It supports some, and you can force others to work with a bunch of hacky workarounds

This is a proper extension store announcement.