Do you mean officially considered beta/stable quality or just that it's there in the Beta or Stable version, so that you don't have to migrate to a less stable version of Firefox in order to use it?

If it's the latter, there's two preferences that you can flip in about:config to enable it (even in Firefox Stable): privacy.userContext.enabled and privacy.userContext.ui.enabled

Mind though that since this feature is currently still under active development, that it might actually be less error-prone in the normally less stable versions of Firefox.