First problem was working out how to launch the addon. It adds a jigsaw piece icon to the toolbar. It must say somewhere in the docs but I had to google to find out.
Biggest problem is that it says it lists 'Unexpired bookmarks'. I wondered if this is a typo for expired but many it checked and offered to remove were valid. EG:
https://www.facebook.com (TypeError: NetworkError when attempting to fetch resource.) [Remove]
Amazon.co.uk: Low Prices in Electronics, Books, Sports Equipment & more (TypeError: NetworkError when attempting to fetch resource.) [Remove]

Other links were, indeed, broken.
Others users have found it works but not here, I'm afraid. Pity.
I would file a bug report but the support link takes me to Github and I don't know how to work it.