adding the button to the toolbar

overlay.js

Application.getExtensions(function (extensions) {
    let extension = extensions.get('[email protected]');

    if (extension.firstRun) {
      var myId    = "enjoyreading-toolbar-button"; // ID of button to add
      var afterId = "urlbar-container";    // ID of element to insert after
      var navBar  = document.getElementById("nav-bar");
      var curSet  = navBar.currentSet.split(",");

      if (curSet.indexOf(myId) == -1) {
        var pos = curSet.indexOf(afterId) + 1 || curSet.length;
        var set = curSet.slice(0, pos).concat(myId).concat(curSet.slice(pos));

        navBar.setAttribute("currentset", set.join(","));
        navBar.currentSet = set.join(",");
        document.persist(navBar.id, "currentset");
        try {
          BrowserToolboxCustomizeDone(true);
        }
        catch (e) {}
      }
    }
})