[avian]

It's a long-standing bug that also affects me. It's quite annoying since it's easy to irreversibly hose up your sync account by going over the remote storage quota. Maybe you don't have a Firefox account and/or sync setup?

There are related issues opened for Temporary Containers, MAC and Firefox and at least from the public discussions not much is happening about it.

[ringer]

I use Temporary containers heavily and the MAC sync is also enabled but I never see anything like this. Sometimes 1-2 container it remains but I can delete it manually. Maybe I'm lucky with this.