Installing extensions in development mode is still possible and I tried that with #PfP. It appears to work perfectly fine, still has all of its data. But users see it under “Not available yet”, they are not allowed to run it.
#Mozilla didn’t bother notifying developers that their add-ons won’t work any more. I didn’t follow the development, so I had to learn about this from a user’s issue report after the new browser went live.