That's because we aren't disabling stripping of the libs.  So they get
stripped before we get to turn them into debuginfo rpms.

Stripping is disabled by default, however the
xpinstall/packager/Makefile strips everything when it builds the
package, regardless of how you configure it.

Change the following line in the RPM spec:
%{__make} MOZILLA_BIN="\$(DIST)/bin/firefox-bin"
to:
%{__make} MOZILLA_BIN="\$(DIST)/bin/firefox-bin" STRIP="/bin/true"
and the problem goes away.

This got fixed yesterday in rawhide.