My job as a freelance translator means I work for different clients who sometimes use the same services, i.e., I have more than one account with those services. Without Multi-Account Containers, my job would be a nightmare. Best add-on ever.
The idea is nice but unless this extension can sync all of your container settings to the Firefox account it's not worth the effort (I can do the same by creating a different profile), especially considering that it often forgets all your containers and you have to set them up from scratch (it's pretty frustrating).
Another missing feature is the ability to open bookmarks in a container quickly. There should be a context menu action for this, e.g. Right click a bookmark > Open in Container > Select a container.
Almost perfect extension for me. It let me separate accounts on sites or group sites... After mess-up with certificates I lost all my custom made preferences and containers. Add back up ability even if I move to other computer and login to my Mozilla account I feel same good Firefox I'm used to.