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.”
It seems there is some limited form of isolation, but not as strong as on Chromium-based browsers:
https://bugzilla.mozilla.org/show_bug.cgi?id=1565196
GrapheneOS claims that Firefox bypasses some work done by them and upstream Android but they don’t specify how exactly…
I’d trust them over myself to understand stuff like that.