After yesterday spending ages deleting TMP containers, today I synced a new install of Firefox on a different, new PC. All TMP containers were recreated. Terrible.
Downgraded from 4 to 1 stars.

A week later I have just deleted all the TMP containers 'synced' to my new machine.
Switched off sync in add-on settings. Closed and reopened Ffox, switched on sync in add-on settings and it put all the TMP containers back in.
Oh and it's now synced a website back into a container I had removed it from on both machines.

Clicking "Remember this decision for this site" or manually changing the site list for a container are ignored or overwritten. It keeps asking though as-if there is any function in any of the controls.

WHYYYYY? What is the intended purpose of any of these functions if all it does is sync back over the top undoing everything I do to try to clean up the mess it keeps replicating?

A community of conscientious supporters' time and energy would be better spent on how it /should/ work, why it is important in theory, and improvements - instead of barely getting it to function as intended and repeatedly cleaning up the mess it spreads everywhere.

A mess admittedly created by Temporary Containers but I haven't had that installed for several years but still have several hundreds of TMP containers to delete. Again.

Good theory.
There is a way to remove the hundreds of synced TMP containers created by Temporary Containers extension, but sync just undoes everything you do, so good luck.
Also ! Watch-out! The page jumps around so if you are trying to use it for what it's for: bulk removal, it's easy to delete ones you wanted to keep.
about:preferences#containers