diff --git a/layout/generic/nsContainerFrame.cpp b/layout/generic/nsContainerFrame.cpp
index 9580a4acfa07..4efb1df9a3e3 100644
--- a/layout/generic/nsContainerFrame.cpp
+++ b/layout/generic/nsContainerFrame.cpp
@@ -1393,10 +1393,7 @@ nsContainerFrame::DeleteNextInFlowChild(nsPresContext* aPresContext,
   }
 
   // Take the next-in-flow out of the parent's child list
-#ifdef DEBUG
-  nsresult rv =
-#endif
-    StealFrame(aPresContext, aNextInFlow);
+  DebugOnly<nsresult> rv = StealFrame(aPresContext, aNextInFlow);
   NS_ASSERTION(NS_SUCCEEDED(rv), "StealFrame failure");
 
 #ifdef DEBUG
diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index 8ab262ec7b88..da9d81989ace 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -878,10 +878,7 @@ nsIFrame::GetUsedMargin() const
   if (m) {
     margin = *m;
   } else {
-#ifdef DEBUG
-    bool hasMargin = 
-#endif
-    StyleMargin()->GetMargin(margin);
+    DebugOnly<bool> hasMargin = StyleMargin()->GetMargin(margin);
     NS_ASSERTION(hasMargin, "We should have a margin here! (out of memory?)");
   }
   return margin;
@@ -956,10 +953,7 @@ nsIFrame::GetUsedPadding() const
   if (p) {
     padding = *p;
   } else {
-#ifdef DEBUG
-    bool hasPadding = 
-#endif
-    StylePadding()->GetPadding(padding);
+    DebugOnly<bool> hasPadding = StylePadding()->GetPadding(padding);
     NS_ASSERTION(hasPadding, "We should have padding here! (out of memory?)");
   }
   return padding;
@@ -5069,10 +5063,7 @@ ComputeOutlineAndEffectsRect(nsIFrame* aFrame,
   uint8_t outlineStyle = outline->GetOutlineStyle();
   if (outlineStyle != NS_STYLE_BORDER_STYLE_NONE) {
     nscoord width;
-#ifdef DEBUG
-    bool result = 
-#endif
-      outline->GetOutlineWidth(width);
+    DebugOnly<bool> result = outline->GetOutlineWidth(width);
     NS_ASSERTION(result, "GetOutlineWidth had no cached outline width");
     if (width > 0) {
       if (aStoreRectProperties) {
diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index 8abfcd9e0248..bcdecb9e0ee0 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -3181,10 +3181,7 @@ void
 nsXULScrollFrame::RemoveHorizontalScrollbar(nsBoxLayoutState& aState, bool aOnBottom)
 {
   // removing a scrollbar should always fit
-#ifdef DEBUG
-  bool result =
-#endif
-  AddRemoveScrollbar(aState, aOnBottom, true, false);
+  DebugOnly<bool> result = AddRemoveScrollbar(aState, aOnBottom, true, false);
   NS_ASSERTION(result, "Removing horizontal scrollbar failed to fit??");
 }
 
@@ -3192,10 +3189,7 @@ void
 nsXULScrollFrame::RemoveVerticalScrollbar(nsBoxLayoutState& aState, bool aOnRight)
 {
   // removing a scrollbar should always fit
-#ifdef DEBUG
-  bool result =
-#endif
-  AddRemoveScrollbar(aState, aOnRight, false, false);
+  DebugOnly<bool> result = AddRemoveScrollbar(aState, aOnRight, false, false);
   NS_ASSERTION(result, "Removing vertical scrollbar failed to fit??");
 }
 
diff --git a/layout/generic/nsImageFrame.h b/layout/generic/nsImageFrame.h
index 3b657577762d..a1b4c9ffe61f 100644
--- a/layout/generic/nsImageFrame.h
+++ b/layout/generic/nsImageFrame.h
@@ -17,6 +17,7 @@
 #include "nsDisplayList.h"
 #include "imgIContainer.h"
 #include "mozilla/Attributes.h"
+#include "mozilla/DebugOnly.h"
 #include "nsIReflowCallback.h"
 
 class nsImageMap;
@@ -327,11 +328,8 @@ private:
     }
 
     void RemoveIconObserver(nsImageFrame *frame) {
-#ifdef DEBUG
-        bool rv =
-#endif
-            mIconObservers.RemoveElement(frame);
-        NS_ABORT_IF_FALSE(rv, "Observer not in array");
+      mozilla::DebugOnly<bool> didRemove = mIconObservers.RemoveElement(frame);
+      NS_ABORT_IF_FALSE(didRemove, "Observer not in array");
     }
 
   private: