diff --git a/browser/base/content/tabview/modules/utils.jsm b/browser/base/content/tabview/modules/utils.jsm
index 96a0e4877828..ab915f12cf76 100644
--- a/browser/base/content/tabview/modules/utils.jsm
+++ b/browser/base/content/tabview/modules/utils.jsm
@@ -666,22 +666,9 @@ let Utils = {
 
   // ----------
   // Function: merge
-  // Merge two arrays and return the result.
+  // Merge two array-like objects into the first and return it.
   merge: function(first, second) {
-    var i = first.length, j = 0;
-
-    if (typeof second.length === "number") {
-      for (let l = second.length; j < l; j++) {
-        first[i++] = second[j];
-      }
-    } else {
-      while (second[j] !== undefined) {
-        first[i++] = second[j++];
-      }
-    }
-
-    first.length = i;
-
+    Array.forEach(second, function(el) Array.push(first, el));
     return first;
   },