diff --git a/gfx/2d/FilterProcessingScalar.cpp b/gfx/2d/FilterProcessingScalar.cpp
index 9e88c563e4..cd9e6f53fa 100644
--- a/gfx/2d/FilterProcessingScalar.cpp
+++ b/gfx/2d/FilterProcessingScalar.cpp
@@ -45,12 +45,12 @@ ApplyMorphologyHorizontal_Scalar(uint8_t* aSourceData, int32_t aSourceStride,
     for (int32_t x = aDestRect.x; x < aDestRect.XMost(); x++, startX++, endX++) {
       int32_t sourceIndex = y * aSourceStride + 4 * startX;
       uint8_t u[4];
-      for (size_t i = 0; i < 4; i++) {
+      for (int32_t i = 0; i < int32_t(std::size(u)); i++) {
         u[i] = aSourceData[sourceIndex + i];
       }
       sourceIndex += 4;
       for (int32_t ix = startX + 1; ix <= endX; ix++, sourceIndex += 4) {
-        for (size_t i = 0; i < 4; i++) {
+        for (int32_t i = 0; i < int32_t(std::size(u)); i++) {
           if (Operator == MORPHOLOGY_OPERATOR_ERODE) {
             u[i] = umin(u[i], aSourceData[sourceIndex + i]);
           } else {
@@ -60,7 +60,7 @@ ApplyMorphologyHorizontal_Scalar(uint8_t* aSourceData, int32_t aSourceStride,
       }
 
       int32_t destIndex = y * aDestStride + 4 * x;
-      for (size_t i = 0; i < 4; i++) {
+      for (int32_t i = 0; i < int32_t(std::size(u)); i++) {
         aDestData[destIndex+i] = u[i];
       }
     }
@@ -97,12 +97,12 @@ static void ApplyMorphologyVertical_Scalar(uint8_t* aSourceData, int32_t aSource
     for (int32_t x = aDestRect.x; x < aDestRect.XMost(); x++) {
       int32_t sourceIndex = startY * aSourceStride + 4 * x;
       uint8_t u[4];
-      for (size_t i = 0; i < 4; i++) {
+      for (int32_t i = 0; i < int32_t(std::size(u)); i++) {
         u[i] = aSourceData[sourceIndex + i];
       }
       sourceIndex += aSourceStride;
       for (int32_t iy = startY + 1; iy <= endY; iy++, sourceIndex += aSourceStride) {
-        for (size_t i = 0; i < 4; i++) {
+        for (int32_t i = 0; i < int32_t(std::size(u)); i++) {
           if (Operator == MORPHOLOGY_OPERATOR_ERODE) {
             u[i] = umin(u[i], aSourceData[sourceIndex + i]);
           } else {
@@ -112,7 +112,7 @@ static void ApplyMorphologyVertical_Scalar(uint8_t* aSourceData, int32_t aSource
       }
 
       int32_t destIndex = y * aDestStride + 4 * x;
-      for (size_t i = 0; i < 4; i++) {
+      for (int32_t i = 0; i < int32_t(std::size(u)); i++) {
         aDestData[destIndex+i] = u[i];
       }
     }