--- a/dom/permission/tests/file_framework.js
+++ b/dom/permission/tests/file_framework.js
@@ -163,21 +163,19 @@ function addPermissions(aPerms, aDoc, aC
   });
   SpecialPowers.pushPermissions(permList, aCallback);
 }
 
 function expandPermissions(aPerms) {
   var perms = [];
   aPerms.forEach(function(el) {
     var access = permTable[el].access ? "readwrite" : null;
-    var expanded = SpecialPowers.unwrap(expand(el, access));
-    // COW arrays don't behave array-like enough, to allow
-    // using expanded.slice(0) here.
+    var expanded = expand(el, access);
     for (let i = 0; i < expanded.length; i++) {
-      perms.push(expanded[i]);
+      perms.push(SpecialPowers.unwrap(expanded[i]));
     }
   });
 
   return perms;
 }
 
 function msgHandler(evt) {
   var data = evt.data;