I have 3 accounts on a website. I thought I could setup 3 different containers, one for each account. But it turns out that switching between containers is not simple. Whenever I tried to switch, I got a pop up from Firefox asking me if I was really sure I wanted to switch my default container for that website. Sometimes it sent me into a redirect loop with the prompt also. I did some googling and found that the consensus was that containers simply aren’t meant to be used that way.
Firefox Multi Account Containers seem to assume you will always use one container for one website, and you can’t easily use multiple containers for a single site. Or at least, it’s very inconvenient to do so.