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.
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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.