I was thinking about how great it would be if I could create a container that I could map to specific URLs, so when I go to certain websites, Firefox will auto open those sites within the correct containers.
This could easily be solved by enabling manual entries of URLs in the "Designated Sites List", when accessed from "Manage this Container > Manage Site List".
Being a privacy junkie, I am not willing to add unverified/potentially unsafe extensions to Firefox, so I don't love the idea of using another browser extension to gain this functionality.