alt Hacker NewsI don't use Nightly and I've been using containers for I think that long without any extension for it.
Edit: looks like I enabled them manually and couldn't remember/find it, it's in about:config under the name "privacy.userContext" instead of "containers".
as far as i remember, the low level container functionality was always in the core browser. what the extension adds is the UI to manage containers. with this change it appears the UI is merged back into the core browser.