If a web page tries to show pop-up windows, Firefox will display a message asking whether to show the pop-ups. (Click the image below for an example.) If these are "hover" menus that appear within the page when you move the mouse cursor, they probably don't open in new windows and will not be blocked by pop-up blockers. They might be hard to use in mobile Firefox because there is no mouse cursor. To open these sub-menus in mobile Firefox, try pressing on the menu. When the sub-menu appears, move your finger away before lifting it.