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?

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.

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.