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