This extension to Multi-Account Containers looks great at first -- its Automatic mode spins up a new container each time you open a new tab, then switches that tab to an existing container, should one be associated with the TLD you visit.
The problem is that Temporary Containers does not delete these temporary containers. They accumulate in the MAC containers list. There is no way to mass delete them, nor set them to auto-delete when the tab is closed.
These WILL sync to your Firefox account and reappear on every machine. Over 3000 temp container made MAC unusably laggy. This was so destructive of my MAC experience that I uninstalled both extensions. Unfortunately, to my knowledge, there is no way to reset the data MAC syncs to Firefox Account.