open Library, on the left you should see "recently bookmarked", click it, press delete

I assume you mean the section near the top of the Bookmarks menu. You can hide it using one of these methods: '''(1) Context Menu''' Windows: Right-click the "Recently Bookmarked" text and choose Hide Recently Bookmarked from the bottom of the right-click context menu. Mac: Ctrl+click the "Recently Bookmarked" text and choose Hide Recently Bookmarked from the bottom of the right-click context menu. <img src="https://support.cdn.mozilla.net/media/uploads/images/2016-09-21-19-16-31-922236.png"> ''Note: A user reported this doesn't work on the Bookmarks Menu on the main menu bar on Mac, only the bookmarks menu that drops from the toolbar button, which is pictured in the screenshot.'' '''(2) Directly Edit a Preference''' The menu item switches a hidden preference from true to false, and you can do that directly as follows: (A) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button promising to be careful. (B) In the search box above the list, type or paste '''book''' and pause while the list is filtered (C) Double-click the '''browser.bookmarks.showRecentlyBookmarked''' preference to switch the value from true to false