I tested this useful addon on Waterfox , but when i click on the icon i don't see the dropdown menu.
(i read somewhere it's by design but by design of what???)
Edit:
With the help of:
https://www.reddit.com/r/waterfox/comments/bkt2t2/multi_account_containers_borked/f0xriwz/ ,
I can see the menu...
Need time to test now!