Containers actually still are integrated in the standard installation of Firefox and can be used even without an addon. You just need to turn them on in about:config:
privacy.userContext.enabled to true
privacy.userContext.ui.enabled to true
privacy.userContext.longPressBehavior to 2
I love using containers with privacy.firstparty.isolate set to true too for extra protection.
This add-on is a GUI to the underlying mechanism. They even say you can choose to use the non-GUI lower level hooks. They think this GUI is a nicer user experience