diff --git a/toolkit/content/Geometry.jsm b/toolkit/content/Geometry.jsm
index 0bac5e7a2d2..1c15ff7060a 100644
--- a/toolkit/content/Geometry.jsm
+++ b/toolkit/content/Geometry.jsm
@@ -343,5 +343,14 @@ Rect.prototype = {
     this.right = f.call(this, this.right);
     this.bottom = f.call(this, this.bottom);
     return this;
+  },
+
+  /** Ensure this rectangle is inside the other, if possible. Preserves w, h. */
+  translateInside: function translateInside(other) {
+    let offsetX = (this.left < other.left ? other.left - this.left :
+        (this.right > other.right ? this.right - other.right : 0));
+   let offsetY = (this.top < other.top ? other.top - this.top :
+        (this.bottom > other.bottom ? this.bottom - other.bottom : 0));
+    return this.translate(offsetX, offsetY);
   }
 };