Also from the official announcement (https://blog.mozilla.org/addons/2023/11/28/open-extensions-on-firefox-for-android-debut-december-14-but-you-can-get-a-sneak-peek-today/ ):
Starting December 14, 2023, extensions marked as Android compatible on addons.mozilla.org (AMO) will be openly available to Firefox for Android users.
“We’ve been so impressed with developer enthusiasm and preparation,” said Giorgio Natili, Firefox Director of Engineering. “Just a few weeks ago it looked like we might have a couple hundred Android extensions for launch, but now we can safely say AMO will have 400+ new Firefox for Android extensions available on December 14. We couldn’t be more thankful to our developer community for embracing this exciting moment.”
You have been able to use a custom repo for extensions for over a year.
Whilst true, this has only been available with Firefox nightly. I’ve found some extensions break the collection where none of the extensions load until you remove the one that breaks it.
It also works on Beta, has for a while.