... does the overlay know where to overlay itself? The IDs of the overlay children and the original XUL are matched to find the target within the files, but the manifest that accompanies the overlay ... locale,install,url,resource:/chrome/xfly/locale/en-US/ These entries tell the chrome registry that, in addition to all of the packages that make up the main Mozilla browser (e.g., the communicator skin in the Modern theme, the en-US locale, ... <menupopup/> of the Tools menu) and their contents are interpolated into the list of children there. This file overlays an extra <menuitem/> into the Tools menu. In this case, the menuitem has...