Yes... I had sessions in those containers, the cookies are the data I guess.

I've had problems where it loses my settings after update. They also don't sync between devices, making them quite cumbersome for me to use (you can copy the settings manually though).

When it's all setup it works great though.

My "data loss" happened when Firefox updated itself automatically, said add-on was not compatible with latest Firefox and was disabled. After some time it became compatible again and was reenabled. All cookies in "containers" were lost.

Not sure anything else was lost though.

Anyway, password manager is still on a global (not per container/account), so it's easy to make mistakes and log in with "credentials A" in "container B". Or am I missing something?

There was/is a button that allowed you to clear your default container's entire session. I almost clicked that.