I mean, I'm not sure why it's not built-in by default!
I was looking for a way to always open a certain bookmark/URL in a certain container (makes sense right?) and only then realised you need the extra add-on (by Mozilla 😃) to make full use of container tabs.
Please integrate the functionality so we can take full advantage of such a great feature!
Link for everyone else: https://addons.mozilla.org/en-US/firefox/addon/multi-account-containers/