Here you can download a build for Mozilla's Gecko SDK for Intel-based Macs (Mozilla doesn't yet have an official release). The build is for a Fink-based libIDL on OS X 10.4.
To install untar the file in a appropriate location. Make sure to update your LD_LIBRARY_PATH to point to the untarred lib directory, if necessary. To get libIDL and glib do
fink install orbit orbit-dev glib