Add layout-debug support for FireFox

browser/extensions/Makefile.in

@@ -43,7 +43,11 @@ VPATH		= @srcdir@
 include $(DEPTH)/config/autoconf.mk
 
 ifneq (,$(findstring inspector,$(MOZ_EXTENSIONS)))
-DIRS  = inspector
+DIRS  += inspector
+endif
+
+ifneq (,$(findstring layout-debug,$(MOZ_EXTENSIONS)))
+DIRS  += layout-debug
 endif
 
 include $(topsrcdir)/config/rules.mk