This is regression since Firefox 50. Reproducible: always Steps To Reproduce: 1. Open Library or History Sidebar 2. Open www.yahoo.com 3. Right click on Yahoo history item in today folder of Library or History Sidebar And Execute "Forget About This Site" 4. Reload(F5) the page 5. Observe, History entry Actual Results: History entry is not updated. No Yahoo entry appears. Expected Results: History entry should be updated. Yahoo entry should appear.