For JAR files, Mozilla requires the contents of content/, locale/language_code/ and skin/ to be displaced by an extra subdirectory named after the package. For example, without a JAR file, you could have this layout in Mozilla's chrome directory:

chrome/evergreen/content/main.xul

but if this were a JAR file:

chrome/evergreen.jar

Then the contents of the jar file would have to look like this:

chrome/content/evergreen/main.xul

In the both cases, the XUL file would be referenced by the chrome URL:

chrome://evergreen/content/main.xul