Just had that this morning too... deleted the bookmarks within and it disappeared from the toolbar. May want to save them to another folder if you want to still keep them, mine were outdated anyway. Now if I can get the css file to move my tabs to the bottom again... life will be good.

Try to set browser.toolbars.bookmarks.2h2020 = false on the about:config page.

• /questions/1320638 Other Bookmarks popped up in Toolbar

You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox