I've been able to use any extension so far on the Nightly version - at least, any that previously worked in Fennec.
But it's an unintuitive process. You need to create an add-on site account, then create a Collection, then add the extensions you want to the collection. Each add-on page has an Add to Collection option, and the collection page itself has a search option but it's a limited subset and so it's kinda useless.
Then you enable some dev mode option in the app, and (manually) copy some account and collection ids into a setting. Then the collection and the add-ons will appear and let you download and install them.
It does feel like a "placate power users while adding as much friction as is reasonably possible" approach, and Mozilla certainly seems to be heading for a greatly restricted add-on experience for all users in the future. There is an abundant feeling of "a PM wants to see the metrics say X" all over this experience, as with most modern Firefox degradations. But at least this is an option for now.
Edit: having to use nightly does suck as well, as I've seen a new different annoying bug every version or two (e.g. broken menus, tap targets not working any more, etc.) that I wouldn't have seen in the main release candidates. But you can at least try to avoid updating whenever you feel the current nightly is ok.