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?