No, on the Release version you have only selected number of addons to choose from.

On Nightly you can create a custom collection of addons on the addons.mozilla.org (AMO) store and provide a link to this collection in settings. This option makes almost every addon from AMO available for you to use on the Nightly version.

Thanks, but I am trying to install custom .xpi files not from AMO store. (say from a github repo) it was possible until Firefox 78, I think. I simply downloaded a zip file and the android browser would recognize it and install the extension.

The latest android browser can't open it.