font-awesome

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  actions: {
    showIcon() {
      this.set('show', true);
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
{{#if show}}
  icon should be here: {{fa-icon "times-circle"}}
{{/if}}
{{outlet}}
<br>
<button {{action "showIcon"}}>showIcon</button>

twiddle.json

{
  "version": "0.11.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.10.2",
    "ember-data": "2.11.0",
    "ember-template-compiler": "2.10.2",
    "ember-testing": "2.10.2"
  },
  "addons": {
    "ember-font-awesome": "2.2.0"
  }
}