I installed the official "Firefox Multi-Account Container Tab" extension which "... is an extension developed by Mozilla that fully optimizes Firefox containers and unlocks its full potential[1]..."
It immediately bombarded me with multiple ads for Mozilla VPN, so I uninstalled it just as immediately.
The model here seems to be nerfing the built-in containers and locking away full functionality behind a confusingly similarly named extension to upsell you on paid services.
Super, super gross.