This add-on's options include opening each (sub)domain in its own container. These containers are temporary: they're deleted a short time after you close their last tab, so you have to log back into each site on each visit. (This may be something you do anyway.)
I don't (yet?) know of an add-on that automatically assigns each domain you visit to its own permanent container, and automatically creates new containers for each new domain.
There's an add-on that does something close to that:
https://addons.mozilla.org/en-US/firefox/addon/temporary-con...
This add-on's options include opening each (sub)domain in its own container. These containers are temporary: they're deleted a short time after you close their last tab, so you have to log back into each site on each visit. (This may be something you do anyway.)
I don't (yet?) know of an add-on that automatically assigns each domain you visit to its own permanent container, and automatically creates new containers for each new domain.