Yes, that's true -- Mozilla only cares about the arches which are in actual widespread use. The fact that Debian provides support for others is great. But I don't use those (nor I know anyone who does) and I have merely provided my own experience (see the subject).
On a side note, I'd like to warn you that the fact of mere availability of a package in Debian for some rare arch doesn't really mean that the program in that package would actually work on that arch just fine. I am e.g. an author of an app which is present in Debian, and mind you, despite the fact that it currently just doesn't work on any big-endian architectures correctly, in Debian it is present for all arches, including the big-endian ones. Of course, there are no bugreports pertaining to those problems -- no one has ever actually tried to use those packages there.