chrome URIs / chrome registry

The DTD file is referenced by a chrome URI, e.g. "chrome://communicator/locale/openLocation.dtd".
All chrome URIs have the form of chrome://<package>/{content|locale|skin}/<path-to-file>.
To which file on disk (or in a .jar package) this URI resolves, is determined by the chrome registry: The chrome registry knows which locales (e.g. en-US, de-AT, it-IT) are installed for which packages (e.g. communicator, navigator, messenger), and which locale is selected for those packages.
As a side note, the chrome registry stores the same information for themes as well.

Example directory tree with files

Sample chrome registry (main chrome.rdf + profile's chrome.rdf)