diff --git a/gfx/thebes/public/gfxPoint.h b/gfx/thebes/public/gfxPoint.h
index 4837235df58..68ea3062f92 100644
--- a/gfx/thebes/public/gfxPoint.h
+++ b/gfx/thebes/public/gfxPoint.h
@@ -141,9 +141,14 @@ struct THEBES_API gfxPoint {
     gfxPoint operator/(const gfxFloat v) const {
         return gfxPoint(x / v, y / v);
     }
+    // Round() is *not* rounding to nearest integer if the values are negative.
+    // They are always rounding as floor(n + 0.5).
+    // See https://bugzilla.mozilla.org/show_bug.cgi?id=410748#c14
+    // And if you need similar method which is using NS_round(), you should
+    // create new |RoundAwayFromZero()| method.
     gfxPoint& Round() {
-        x = NS_round(x);
-        y = NS_round(y);
+        x = NS_floor(x + 0.5);
+        y = NS_floor(y + 0.5);
         return *this;
     }
 };
diff --git a/gfx/thebes/public/gfxRect.h b/gfx/thebes/public/gfxRect.h
index a283bb5c017..1971bf7238f 100644
--- a/gfx/thebes/public/gfxRect.h
+++ b/gfx/thebes/public/gfxRect.h
@@ -115,10 +115,15 @@ struct THEBES_API gfxRect {
         Outset(sides[0], sides[1], sides[2], sides[3]);
     }
 
-    // Round the rectangle to integer coordinates; rounds to the neearest
-    // pixel centers. Suitable for most places where integral device coordinates
+    // Round the rectangle to integer coordinates.
+    // Suitable for most places where integral device coordinates
     // are needed, but note that any translation should be applied first to
-    // avoid pixel rounding errors
+    // avoid pixel rounding errors.
+    // Note that this is *not* rounding to nearest integer if the values are negative.
+    // They are always rounding as floor(n + 0.5).
+    // See https://bugzilla.mozilla.org/show_bug.cgi?id=410748#c14
+    // If you need similar method which is using NS_round(), you should create
+    // new |RoundAwayFromZero()| method.
     void Round();
 
     // grabbing specific points
diff --git a/gfx/thebes/src/gfxRect.cpp b/gfx/thebes/src/gfxRect.cpp
index b7ece07093f..88da6a31200 100644
--- a/gfx/thebes/src/gfxRect.cpp
+++ b/gfx/thebes/src/gfxRect.cpp
@@ -76,10 +76,11 @@ gfxRect::Union(const gfxRect& aRect) const
 void
 gfxRect::Round()
 {
-    gfxFloat x0 = NS_round(X());
-    gfxFloat y0 = NS_round(Y());
-    gfxFloat x1 = NS_round(XMost());
-    gfxFloat y1 = NS_round(YMost());
+    // Note that don't use NS_round here. See the comment for this method in gfxRect.h
+    gfxFloat x0 = NS_floor(X() + 0.5);
+    gfxFloat y0 = NS_floor(Y() + 0.5);
+    gfxFloat x1 = NS_floor(XMost() + 0.5);
+    gfxFloat y1 = NS_floor(YMost() + 0.5);
 
     pos.x = x0;
     pos.y = y0;