No, for now you can't install a custom extension this way.

In the release version you can only choose those from the list in menu -> Addons.

Thanks.

This isn't a good news at all for me and my work team. We're using a custom developed extension for work (to automate specific tasks to client's servers).

We'll have to downgrade to the last version of freedom :(

Anyone does know: - what is the last version where custom apk installation was possible ? (I ckecked the changelog but it was limited to a certain amount of versions) - is there any fork that disable this limitation ?

I'll look to an open issue for it; I'm super surprised of this big freedom limitation in an open source project :(

I am sorry, but there is not going to be a positive answer to either of those points.

Even if you did sideload (never recommended) an older version, you would not get ongoing security and stability updates.

I am not aware of any fork that you could use that would support your add-on.

If the add-on was deployed from the Mozilla add-on site, there is a way that you could deploy it to a Nightly version of Firefox for Android. This approach is designed to assist add-on developers test add-ons prior to support for add-ons being widened in future updates.