[Pale-Moon] make sure anchor.top is not smaller than zero

palemoon/modules/FormValidationHandler.jsm

@@ -124,7 +124,7 @@ var FormValidationHandler =
     let tabBrowser = aWindow.gBrowser;
     this._anchor = tabBrowser.popupAnchor;
     this._anchor.left = aPanelData.contentRect.left;
-    this._anchor.top = aPanelData.contentRect.top;
+    this._anchor.top = Math.max(0, aPanelData.contentRect.top);
     this._anchor.width = aPanelData.contentRect.width;
     this._anchor.height = aPanelData.contentRect.height;
     this._anchor.hidden = false;