WebExtension: Traversal permission denied to access the "target" object

Using FF WebExtension I am trying to clone an element from a website that I don't have. This is what I am using (taken from W3C ).

// Get the last <li> element ("Milk") of <ul> with id="myList2"
var itm = document.getElementById("myList2").lastChild;

// Copy the <li> element and its child nodes
var cln = itm.cloneNode(true);

// Append the cloned <li> element to <ul> with id="myList1"
document.getElementById("myList1").appendChild(cln); 

      

        
        
        
      

    

However, this error appears:

Error: permission to access object "target"

Here is my manifest:

{

  "manifest_version": 2,
  "name": "w",
  "version": "0.1",

  "description": "w",

  "icons": {
    "48": "icons/border-48.png"
  },
  "content_scripts": [
    {
      "matches": ["http://localhost/*"],
      "js": ["rh-addbutton.js","rh-addpage.js"]
    }
  ],
  "permissions": ["http://localhost/*"]

}

      

        
        
        
      

    

Am I missing permission? I cannot find the correct permission on MDN.