Access Firefox' LocalStorage from extension

Question

I build a Firefox Extension with an options page using this documentation Implement a settings page

On this settings page, the user has the possibility to enter Username and Password which then are stored in localStorage.

function saveOptions() {

localStorage.setItem("hbz_username", document.querySelector("#username").value);
localStorage.setItem("hbz_pass", Aes.Ctr.encrypt(document.querySelector("#password").value, '12345678910111213', 128));

}

The addon manipulates the DOM of a website and generates links to a REST Webservice within HTML elements which are containing a string that matches a specific Regex pattern. When one clicks on that link, the extension should load the user credentials from localStorage

var username= localStorage.getItem("hbz_username");
var passwd = localStorage.getItem("hbz_pass");

But this doesn't work. When I'm trying it the way described in the above listed documentation, I get a "browser is not defined" Error in the F12 Console.

function restoreOptions() {

function setCurrentChoice(result) {
username = result.username;
}

function onError(error) {
console.log(`Error: ${error}`);
}

var getting = browser.storage.local.get("hbz_username");
getting.then(setCurrentChoice, onError);
}

So my question is: How do I access the localStorage from my extension?

---

Edit: Added the contents of my previous answer on the post to the question.

Sorry for the wait but I had to prepare a minified version of the plugin without any cryptography or such.

package.json

{
"title": "Addon From Scratch",
  "name": "addon-from-scratch",
  "version": "0.0.1",
  "description": "A basic add-on",
  "main": "index.js",
  "author": "Chris",
  "engines": {
    "firefox": ">=38.0a1",
    "fennec": ">=38.0a1"
  },
  "license": "MIT",
  "keywords": [
    "jetpack"
  ]
}

index.js

var self = require("sdk/self");

// a dummy function, to show how tests work.
// to see how to test this function, look at test/test-index.js
function dummy(text, callback) {
  callback(text);
}

exports.dummy = dummy;

var tag = "body"
var buttons = require('sdk/ui/button/action');
var tabs = require("sdk/tabs");
var pageMod = require("sdk/page-mod");
var data = require("sdk/self").data;


var button = buttons.ActionButton({
  id: "mozilla-link",
  label: "Visit Mozilla",
  icon: {
    "16": "./icon-16.png",
    "32": "./icon-32.png",
    "64": "./icon-64.png"
  },
  onClick: handleClick
});

function handleClick(state) {
  tabs.open("login.html")
}

function onOpened() {
  console.log(`Options page opened`);
}

function onError(error) {
  console.log(`Error: ${error}`);
}

pageMod.PageMod({
  include: "*",
  contentScriptFile: data.url("modify-content.js"),
  onAttach: function(worker) {
    worker.port.emit("getElements", tag);
    worker.port.on("gotElement", function(elementContent) {
      console.log(elementContent);
    });
  }
});

modify-content.js

self.port.on("getElements", function() {


    var username= localStorage.getItem("hbz_username");
    var passwd = localStorage.getItem("hbz_pass");
    //here is where usually the DOM Manipulation takes place and the link to the REST Service is created. The link contains the username and the encrypted password as param
    alert(username + passwd);


});

options.js

function saveOptions() {

    localStorage.setItem("hbz_username", document.querySelector("#username").value);
    localStorage.setItem("hbz_pass", document.querySelector("#password").value);

}

function restoreOptions() {

  function setCurrentChoice() {
    document.querySelector("#username").value = localStorage.getItem("hbz_username");
    document.querySelector("#password").value = localStorage.getItem("hbz_pass");
  }

  function onError(error) {
    console.log(`Error: ${error}`);
  }

  setCurrentChoice();
}

document.addEventListener("DOMContentLoaded", restoreOptions);
document.querySelector("form").addEventListener("submit", saveOptions);

login.html

<form>
Username:<input type="text" id="username" />
Password:<input type="password" id="password" />
<input type="submit" Value="Submit" />
</form> 
<script src="options.js"></script>

As I mentioned, I already tried the solution from the above mentioned documentation. This is the code as it is actually. I hope this helps.