But Firefox also has browser profiles now (something I wanted for so long I moved back to chrome to get it), so what’s the purpose of first-party containers now? Don’t browser profiles provide a more robust, very visible delineation between different accounts/user profiles?
A profile is a full new browser, these containers are just tabs. So they mix into the current browser window.
They are much more lightweight, and the workflow is better, just switch tabs.
Firefox has had profiles for a long time (via about:profiles and a command-line argument). Unfortunately, the new profiles are not compatible with the old ones and cannot be migrated.