Related: Multi Account Containers - Same thing as the facebook container but more generi - https://addons.mozilla.org/en-US/firefox/addon/multi-account... Temporary Containers - Same container concept but you can create and destroy them on the fly - https://addons.mozilla.org/en-US/firefox/addon/temporary-con...
There's a function to always open a site in a container, but this obviously falls over if you have multiple accounts (aws, gmail, etc), plus the UI is poorly communicated.
There are requests to make it so you can associate a container with a bookmark, for instance, but apparently Moz prevented Moz from doing this via browser apis, and have been powerless to do anything about it for years.
It could have been awesome.