title: Do firefox addons have something similar to "background page" concept?
body: <p>I worked with Chrome extensions which have so called background page - an html page that is loaded in background once per browser window. You can store there some javascript variables, can access extension's own localstorage, can communicate back and force with content scripts (scripts injected to pages).</p>
  
  <p>Is there anything similar in Firefox and how do I use it for the tasks listed above?</p>