Firefox

This isn't true anymore. Any extensions that are explictly marked as Android compatible (by the author) should now be installable from AMO. I'm pretty sure that the extension needs to use manifest v3 to be able to be Android compatible, but as long as that is true and the author has set the Android compatibility flag, you should be able to add it normally.

Installing from AMO seems to work fine for me - although I'm using Nightly variant, which certainly could behave differently. However, Firefox 120 is supposed to work the same I believe.