cursor-menu mismatch

Hi, Since a couple of days, the dropdown menu for my bookmarks is at first displayed below 'Bookmarks' but when moving the cursor over it, the menu jumps to the lefthand corner of the screen. Moving the cursor next to the menu (i.e. not over it!) makes submenu's appear with a gap between them and the first menu. Clicking does open websites, but awkward it is. What shall I do, please? Thanks, best regards, David