Love this add-on. It should be built in (currently there is a builtin one but that is limited at what it can do).
4 missing features will make this add-on as close to perfect as it can get.

1. Restricted container so that ONLY the sites specified to open in a container opens in that container. All other sites open in default container. For example - When this setting is on, clicking a link in a container should open the new site in the default container (or its own container if there is any) instead of opening in current container. So clicking a non facebook/google link in a container specified for Facebook/Google will open in the default container.
2. Backup/Sync containers so that when Firefox Sync is in use all containers and settings will automatically propagate to all instances of Firefox in use.
3. Ability to move a currently opened tab into a container.
4. (Too far-fetched) Container for Firefox iOS/Android.