It seems like the mozilla-devel package ought to include the XPIDL
Through some silliness on my part, I though that xpidl wasn't getting installed
by the RPMs at all. It is, in fact. However, it's installed by the mozilla RPM
rather than the mozilla-devel RPM. Since it should only be needed when
compiling, it probably is better off in mozilla-devel, so I'll leave this open.
Fixed in rawhide.