They are in Bookmarks - Everything in the bookmarks bar is in Bookmarks. Just go to the Bookmarks manager and move the folder where you want it - from the Bookmarks bar to ... well, wherever you think it should be (generally under "Other bookmarks" somewhere).
Replicated online incremental and fresh install only working for user that installed it on macOS 10.13.6 so used admin user to trash Opera 56.0 bundle followed by trying offline installer => https://www.opera.com/download/get/?partner=www&opsys=MacOS for copy of Opera 56.0.3051.36 bundle into Applications folder, which works for a non-admin user.