The 'Manage Site List' feature is relatively new, it wasn't there in earlier versions. Also, the UI is... not great. For example you cannot add new urls here.

The 'Manage Site List' feature is relatively new, it wasn't there in earlier versions. Also, the UI is... not great. For example you cannot add new urls here. Some websites forward you to some other external url through another domain for example. there is no way to add these custom domains. So annoying. There yet another extension to solve that problem, but... it doesn't support Firefox 'sync'.