12.1. Overlays
The overlay system is quite simple. One XUL document is the master document. This document provides a starting point for the final content. Any other XUL documents are overlays. Overlay content is merged into, or added to, the master document's content. This happens in memory when those documents are loaded and has no effect on the original files.
An overlay is a XUL document based on the <overlay> tag instead of the <window> tag. Such a file has a .xul extension and is well-formed XML, but it isn't meant to be displayed alone. Mozilla can display an overlay file by itself, but that is only useful for testing purposes.
Mozilla also supports stylesheet overlays. These are plain Mozilla CSS2 files with a .css file extension. They ...
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