Profiles contain all your settings, history, bookmarks, themes, sync settings, etc. They’re stored in a folder on disk. It’s kinda a hassle to switch between them.

Containers isolate cookies (and other stuff?) within a single tab or group of tabs. It lets you run a specific website or set of websites in isolation from your other browsing activity. This has security benefits, but is also great for logging into the same service with multiple accounts simultaneously.

I was initially annoyed by Firefox profiles because they’re clunkier than Chrome accounts, but was very happy when I figured out that Firefox Containers gave me the isolation benefits I was actually looking for, with some real improvements over Chrome.