I'm trying to understand what possible difference it makes. Yes, in the theoretical world where everything is perfect, we'd only have one language to fill the whole that Python/Perl/Ruby fills. But even in that world, if it had to interact with this world and its variety of graphic formats, I still think libtiff would likely be one of the basic support packages that would get pulled in when building all the libraries that Mozilla depends on. There is no reason that libgtk should be limited to the features that Mozilla needs, or that Mozilla shouldn't depend on it because it provides a feature that Mozilla doesn't need. Libraries just don't work that way.
Back to Perl and Python ... so? What's the cost? If you're talking about a tiny embedded system where only supporting one is a major space saver, then I understand the frustration, but if you're compiling Mozilla, both Perl and Python are negligible in size.