--- a/layout/style/ServoBindings.cpp
+++ b/layout/style/ServoBindings.cpp
@@ -1719,16 +1719,19 @@ ShutdownServo()
 
 GeckoFontMetrics
 Gecko_GetFontMetrics(RawGeckoPresContextBorrowed aPresContext,
                      bool aIsVertical,
                      const nsStyleFont* aFont,
                      nscoord aFontSize,
                      bool aUseUserFontSet)
 {
+  // This function is still unsafe due to frobbing DOM and network
+  // off main thread. We currently disable it in Servo, see bug 1356105
+  MOZ_ASSERT(NS_IsMainThread());
   MutexAutoLock lock(*sServoFontMetricsLock);
   GeckoFontMetrics ret;
   // Safe because we are locked, and this function is only
   // ever called from Servo parallel traversal or the main thread
   nsPresContext* presContext = const_cast<nsPresContext*>(aPresContext);
   presContext->SetUsesExChUnits(true);
   RefPtr<nsFontMetrics> fm = nsRuleNode::GetMetricsFor(presContext, aIsVertical,
                                                        aFont, aFontSize,