--- a/widget/gtk/nsLookAndFeel.cpp
+++ b/widget/gtk/nsLookAndFeel.cpp
@@ -70,16 +70,17 @@ ApplyColorOver(const GdkRGBA& aSource, G
     gdouble destCoef = aDest->alpha * (1.0 - sourceCoef);
     gdouble resultAlpha = sourceCoef + destCoef;
     if (resultAlpha != 0.0) { // don't divide by zero
         destCoef /= resultAlpha;
         sourceCoef /= resultAlpha;
         aDest->red = sourceCoef * aSource.red + destCoef * aDest->red;
         aDest->green = sourceCoef * aSource.green + destCoef * aDest->green;
         aDest->blue = sourceCoef * aSource.blue + destCoef * aDest->blue;
+        aDest->alpha = resultAlpha;
     }
 }
 
 static void
 GetLightAndDarkness(const GdkRGBA& aColor,
                     double* aLightness, double* aDarkness)
 {
     double sum = aColor.red + aColor.green + aColor.blue;