This is a very annoying error with the latest version of Firefox under Windows 7, which is also up to date as are all my drivers. The problem is that whenever a drop-down dialog or menu gets opened, the main Firefox window (containing the open web page) gets redrawn on top of it, so all you see is the shadow beneath the menu. You can see the results yourself here, where I've taken a screenshot of me attempting to open the Bookmarks menu. (The tabs look strange because I use the Tree Style Tab add-on): http://flic.kr/p/b7n8TR