I think the menu in your toolbar that allows you to open a new tab in a particular container (you then have to do it via a long press or right-click on the new tab button), to re-open a current tab in a different container, and this new container-specific VPN functionality.

Without the extension, you can't set a domain to always open in a particular container, at least I couldn't find a UI to do so. To me, that is a very important part of using containers.

You extension also lets you sort open tabs by container but that doesn't sound very useful.

I experienced the same thing. Without the extension but with these flags enabled, there's no way to bind certain hostnames to certain containers.

This is sort of documented in https://support.mozilla.org/en-US/kb/containers#w_what-you-c... #5. That page covers it but doesn't clearly spell out that the itemized features are only possible with the extension. As far as I can tell, the extension is called "Multi-Account Containers" while the feature is called "containers" and readers are expected to know the difference when reading that doc. (Suggestion to Firefox docs maintainers: if that's correct, change all occurrences of "Multi-Account Containers" to "Multi-Account Containers Extension")

Oh, I had never discovered that feature... So I won't miss it yet. Need to think whether that makes it worth to use the extension again. For now I just enabled the manual configuration and have not noted any differences yet.

Nothing. The containers support is in the WebExtension API.

Firefox's multi-account container extension has no special access, beyond usual extension permissions.