Very useful, because if you tell Firefox to always open a URL in a specific container it will open all URLs of that site (!) in that container, like all google.com products (Gmail, Drive, etc.). With this add-on, you can separate, for example, Gmail (in one container) from Drive (in a different container).

Only one flaw: no favicon support. Links changed to work with this add-on get a generic symbol by Firefox: a globe (like for sites without favicons).