--- a/layout/style/nsStyleContext.cpp
+++ b/layout/style/nsStyleContext.cpp
@@ -214,16 +214,20 @@ nsStyleContext::CalcStyleDifference(nsSt
   // nobody ever looked at that struct's data).  In other words, we
   // can't skip later structs if we get a big change up front, because
   // we could later get a small change in one of those structs that we
   // don't want to miss.
 
   DebugOnly<uint32_t> structsFound = 0;
 
   if (IsGecko()) {
+    // CalcStyleDifference is always called on the main thread for Gecko
+    // style contexts.  This assertion helps the heap write static analysis.
+    MOZ_ASSERT(NS_IsMainThread());
+
     // FIXME(heycam): We should just do the comparison in
     // nsStyleVariables::CalcDifference, returning NeutralChange if there are
     // any Variables differences.
     const nsStyleVariables* thisVariables = PeekStyleVariables();
     if (thisVariables) {
       structsFound |= NS_STYLE_INHERIT_BIT(Variables);
       const nsStyleVariables* otherVariables = aNewContext->StyleVariables();
       if (thisVariables->mVariables == otherVariables->mVariables) {