I use the extension "Firefox Multi-Account Containers" extensively. I have created many containers, and have assigned many different websites to different containers, often some sites sharing a single container.
When I want to set "Always Open This Site in ..." for a new website, I have to scroll the long, unsorted list of containers. There is no search field I can use, and there is no categorisation I can utilise.
So I have the feature requests:
Provide a search field for existing containers in the "Always Open This Site in ..." function,
allow to create categories like a folder hirarchy for containers, which can be collapsed/ expanded individually.