--- a/browser/components/preferences/in-content/main.js
+++ b/browser/components/preferences/in-content/main.js
@@ -1071,19 +1071,19 @@ EngineStore.prototype = {
     }
 
     let engineName = aEngine.name;
     let index = this._engines.findIndex(element => element.name == engineName);
 
     if (index == -1)
       throw new Error("invalid engine?");
 
-    this._engines.splice(index, 1);
+    let removedEngine = this._engines.splice(index, 1)[0];
 
-    if (this._defaultEngines.some(this._isSameEngine, this._engines[index]))
+    if (this._defaultEngines.some(this._isSameEngine, removedEngine))
       gMainPane.showRestoreDefaults(true);
     gMainPane.buildDefaultEngineDropDown();
     return index;
   },
 
   restoreDefaultEngines() {
     var added = 0;