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In order to generate an .xpt file from an .idl file in recent versions of the Gecko \ XUL Runner SDK in Windows, you need to use the typelib.py script. Assuming you installed your XULRunner SDK at <MOZSDKDIR>, you then need to issue a command similar to
C:\working-dir> <MOZSDKDIR>\sdk\bin\typelib.py <InputFile.idl> -o <OutputFile.xpt> -I <MOZSDKDIR>\idl