var folder = Cc['@mozilla.org/file/local;1']
              .createInstance(Ci.nsILocalFile);
folder.initWithPath('C:\\Program Files\\MyCommonLibs\\Modules');
var folderUrl = ioService.newFileURI(folder);

var Cc['@mozilla.org/network/io-service;1']
     .getService(Ci.nsIIOService);
var ResProtocolHandler = IOService.getProtocolHandler('resource')
       .QueryInterface(Ci.nsIResProtocolHandler);

ResProtocolHandler.setSubstitution('mymodules', folderUrl);