I don't know whether it's a forum question or a bug. In xulrunner-1.8, there was xulrunner--xpcom.pc, certain macro (in dev-util/devhelp) that checks for gecko 1.9 suggests that such file should be in xulrunner-1.9 too. Is that assumption invalid or is this file really missing ?
http://fedoraproject.org/wiki/Releases/FeatureXULRunnerAPIChanges