Actually you can. You need nightly and you need to setup some account and set your extensions there. This is just so they can show you that they still work and they have disabled them because reasons. Nicely played.

I'm not sure I understand, you mean I need to use nightly and I need to publish the extensions to AMO myself? Neither of that is going to happen.

You don't need to publish the extensions, but you need to choose the extensions that you want to make available to Firefox. The instructions are here:

https://blog.mozilla.org/addons/2020/09/29/expanded-extensio...

The stable version of Fennec F-Droid (a fork of Firefox for Android) also supports custom extensions through this method if you don't want to use Firefox Nightly:

https://f-droid.org/en/packages/org.mozilla.fennec_fdroid/

I want to use a non AMO extension