Firefox needs idldir as well.

--- mozilla/build/unix/mozilla-xpcom.pc.in	2005-09-30 04:38:08.000000000 +0200
+++ mozilla.az/build/unix/mozilla-xpcom.pc.in	2005-09-30 04:38:29.000000000 +0200
@@ -2,6 +2,7 @@
 exec_prefix=%exec_prefix%
 libdir=%libdir%
 includedir=%includedir%
+idldir=%idldir%
 
 Name: XPCOM
 Description: The Mozilla Cross Platform Component Library