After installing Firefox 57 aka Firefox Quantum, you can notice a bookshelf icon on the toolbar, its the library panel, which has all your stuff: Bookmarks, Pocket list, History, Downloads, Synced Tabs, Screenshots and Recent Highlights.
Items in Recent Highlights in Library menu are populated by the data from Activity Stream in the New Tab Page. If you want to hide or remove it, you can, here is how that can be done.
Removing Recent highlights in the Library menu
- Visit about:config
- Type ‘Activity’, find the preference ‘browser.library.activity-stream.enabled’ and change its value to’ false’.