Having separate cookies for each container is an absolute godsend!
The thing I really miss is, that you aren't able to set containers in bookmarks, that's why I deducted a star.

On YouTube, I have a container for each of my channels, so I don't have to switch accounts constantly, and each container is saved as a separate bookmark, so I can access that channel easily.

For the time being I installed a separate extension to solve this, but imho, this should definitely be solved in this extension, since this is a very basic/common request I would imagine.
https://addons.mozilla.org/en-US/firefox/addon/container_bookmarks/