diff --git a/mozilla/layout/generic/nsLineLayout.cpp b/mozilla/layout/generic/nsLineLayout.cpp
index 8bede05d2f3..7f75e8c9fc7 100644
--- a/mozilla/layout/generic/nsLineLayout.cpp
+++ b/mozilla/layout/generic/nsLineLayout.cpp
@@ -2839,7 +2839,9 @@ nsLineLayout::HorizontalAlignFrames(nsRect& aLineBounds,
             if (numSpaces > 0) {
               FrameJustificationState state = { numSpaces, numLetters, remainingWidth, 0, 0, 0, 0, 0 };
 
-              ApplyFrameJustification(psd, &state);
+              // Apply the justification, and make sure to update our linebox
+              // width to account for it.
+              aLineBounds.width += ApplyFrameJustification(psd, &state);
             }
           }
         }
diff --git a/mozilla/layout/generic/nsLineLayout.h b/mozilla/layout/generic/nsLineLayout.h
index f6e80e08d1d..2bc6f513eaf 100644
--- a/mozilla/layout/generic/nsLineLayout.h
+++ b/mozilla/layout/generic/nsLineLayout.h
@@ -527,7 +527,11 @@ protected:
     nscoord mWidthForSpacesProcessed;
     nscoord mWidthForLettersProcessed;
   };
-  nscoord ApplyFrameJustification(PerSpanData* aPSD, FrameJustificationState* aState);
+
+  // Apply justification.  The return value is the amount by which the width of
+  // the span corresponding to aPSD got increased due to justification.
+  nscoord ApplyFrameJustification(PerSpanData* aPSD,
+                                  FrameJustificationState* aState);
 
 
 #ifdef DEBUG