Using FF 72.0 & Multi-Account Containers 6.1.1 it lasted for about 15 mins then broke. I cleared all my history including the cache and after that container assignment in the URL tab disappeared forever. It previously appeared using a mouse right click on the tab. Tried bouncing everything and reinstalling it...still dead. I can manage the containers but putting a tab in one is gone. Finally got it working by completely reseting FF and reinstalling it along with my other add-ons.