@ricci But.. It's in the core, I did not install anything, and I have used these containers for multiple years now.
What I did install is the Facebook Container that _automatically_ encloses all Meta websites in a separate container, so they don't get cross-site data: https://addons.mozilla.org/en-US/firefox/addon/facebook-container/
@zverik Yeah, I'm confused, a few other people have responded with comments like yours. But I just tried starting up a fresh profile (so no add-ons or anything) and as far I could tell there was nothing related to containers in the UI: no list of them, no buttons to make a new one, just the usual 'open new tab' (no 'open tab in container X') so I'm not sure. I wonder if it comes pre-installed in some cases somehow?