Disable test_contextmenu.html on Linux

browser/base/content/test/Makefile.in

@@ -48,8 +48,6 @@ _TEST_FILES = \
 		feed_discovery.html \
 		test_bug395533.html \
 		bug395533-data.txt \
-		test_contextmenu.html \
-		subtst_contextmenu.html \
 		ctxmenu-image.png \
 		video.ogg \
 		test_offlineNotification.html \
@@ -73,6 +71,16 @@ _TEST_FILES = \
 		test_bug452451.html \
 		$(NULL)
 
+ifeq (,$(filter gtk2,$(MOZ_WIDGET_TOOLKIT)))
+_TEST_FILES += \
+		test_contextmenu.html \
+		subtst_contextmenu.html \
+		$(NULL)
+
+else
+$(warning test_contextmenu.html disabled because of intermittent failures; bug 513558)
+endif
+
 # The following tests are disabled because they are unreliable:
 #   browser_bug423833.js is bug 428712
 #   browser_sanitize-download-history.js is bug 432425