My one wish is if you could set a default container. I use FF to browser Facebook primarily and want that to be default, but instead I have to load Firefox then load a container then load Facebook. Wish it were just open FF -> Facebook is homepage.
There is also a Facebook-specific container addon that limits Facebook's footprint even more than the standard container addon (and will work alongside it)
Also with the standard container addon you can specify that sites always open only in your specified container, so you could just set Facebook to always open in its own container that way.