diff --git a/core/flyout_vertical.js b/core/flyout_vertical.js
index 968d01c78..c95de2607 100644
--- a/core/flyout_vertical.js
+++ b/core/flyout_vertical.js
@@ -155,6 +155,9 @@ Blockly.VerticalFlyout.prototype.position = function() {
   var x = targetWorkspaceMetrics.absoluteLeft;
   if (this.toolboxPosition_ == Blockly.TOOLBOX_AT_RIGHT) {
     x += (targetWorkspaceMetrics.viewWidth - this.width_);
+    // Save the location of the left edge of the flyout, for use when Firefox
+    // gets the bounding client rect wrong.
+    this.leftEdge_ = x;
   }
   this.positionAt_(this.width_, this.height_, x, y);
 };
@@ -315,6 +318,30 @@ Blockly.VerticalFlyout.prototype.getClientRect = function() {
     return new goog.math.Rect(x - BIG_NUM, -BIG_NUM, BIG_NUM + width,
         BIG_NUM * 2);
   } else {  // Right
+    // Firefox sometimes reports the wrong value for the client rect.
+    // See https://github.com/google/blockly/issues/1425 and
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=1066435
+    if (goog.userAgent.GECKO &&
+        this.targetWorkspace_ && this.targetWorkspace_.isMutator) {
+      // The position of the left side of the mutator workspace in pixels
+      // relative to the window origin.
+      var targetWsLeftPixels =
+          this.targetWorkspace_.svgGroup_.getBoundingClientRect().x;
+      // The client rect is in pixels relative to the window origin.  When the
+      // browser gets the wrong value it reports that the flyout left is the
+      // same as the mutator workspace left.
+      // We know that in a mutator workspace with the flyout on the right, the
+      // visible area of the workspace should be more than ten pixels wide.  If
+      // the browser reports that the flyout is within ten pixels of the left
+      // side of the workspace, ignore it and manually calculate the value.
+      if (Math.abs(targetWsLeftPixels - x) < 10) {
+        // If we're in a mutator, its scale is always 1, purely because of some
+        // oddities in our rendering optimizations.  The actual scale is the
+        // same as the scale on the parent workspace.
+        var scale = this.targetWorkspace_.options.parentWorkspace.scale;
+        x = x + this.leftEdge_ * scale;
+      }
+    }
     return new goog.math.Rect(x, -BIG_NUM, BIG_NUM + width, BIG_NUM * 2);
   }
 };