Sliding Show all Bookmark window is appearing and splitting all web pages (see screenshot).

Can open bookmarks when I want thru Firefox menu. Problem is the opened Show All Bookmarks sub menu is appearing in the lower half of all Firefox web pages that I open, splitting the page. I can slide this bookmark page down to make it disappear or up to cover the web page. Am looking for a solution to prevent it from appearing in Firefox at all unless I click on bookmarks it in the menu bar to get the drop down menu. Your help would be appreciated.