It's good start.

The interface could be improved. It's very clunky to add and manage new sites in containers. You can't add currently opened tab website to a group. You can't manually add or edit websites. You first have to open a new container then load the website then tick "always open in ___" if you want it loaded as default. There's really no options to speak of. You can change the name, colors and icons of containers and that's about it. Opening a link in the same container designated for facebook that embeds facebook trackers kind of defeats the privacy. Making the container exclusive to the specified sites as option could help.

As much as I find this addon useful, it's developed by Mozilla, It's no Screenshot add-on. I imagine it will eventually meet the same fate as Tab Groups.