I've been using containers for a while, and have to say that UX wise, it needs work, it's too easy to inadvertently leave a container tab, and it's easy to get them confused.

The effect is "invisible", so there's no warning sign when you've done something wrong. My use-case is managing instagram profiles which periodically log you out, so if you forget and log in with the wrong one it's kind of game over.

In sum, it's too wonky for the general masses. Sure, Mozilla could add it just for the experts but that doesn't seem to be their approach which I can respect. After all it's not hard for an "expert" to get containers running.