diff --git a/layout/style/nsStyleContext.cpp b/layout/style/nsStyleContext.cpp
index 626cef0d47ce..b6cf5aaa3e2c 100644
--- a/layout/style/nsStyleContext.cpp
+++ b/layout/style/nsStyleContext.cpp
@@ -1148,7 +1148,10 @@ nsStyleContext::CalcStyleDifferenceInternal(StyleContextLike* aNewContext,
       const nsStyleOutline *thisVisOutline = thisVis->StyleOutline();
       const nsStyleOutline *otherVisOutline = otherVis->StyleOutline();
       bool haveColor;
-      nscolor thisColor, otherColor;
+      // Dummy initialisations to keep Valgrind/Memcheck happy.
+      // See bug 1289098 comment 1.
+      nscolor thisColor = NS_RGBA(0, 0, 0, 0);
+      nscolor otherColor = NS_RGBA(0, 0, 0, 0);
       if (thisVisOutline->GetOutlineInitialColor() !=
             otherVisOutline->GetOutlineInitialColor() ||
           (haveColor = thisVisOutline->GetOutlineColor(thisColor)) !=
diff --git a/layout/style/nsStyleStruct.cpp b/layout/style/nsStyleStruct.cpp
index b699c7729ddd..0f742845b7d8 100644
--- a/layout/style/nsStyleStruct.cpp
+++ b/layout/style/nsStyleStruct.cpp
@@ -3698,7 +3698,10 @@ nsStyleTextReset::CalcDifference(const nsStyleTextReset& aNewData) const
   }
 
   // Repaint for decoration color changes
-  nscolor decColor, otherDecColor;
+  // Dummy initialisations to keep Valgrind/Memcheck happy.
+  // See bug 1289098 comment 1.
+  nscolor decColor = NS_RGBA(0, 0, 0, 0);
+  nscolor otherDecColor = NS_RGBA(0, 0, 0, 0);
   bool isFG, otherIsFG;
   GetDecorationColor(decColor, isFG);
   aNewData.GetDecorationColor(otherDecColor, otherIsFG);