title: How to start Service Worker in Firefox?
body: <p>I am trying to learn how to make Service Worker and while it is working as intended in Chrome, for some unknown reason it is not working in Firefox v56. Here is the demo that should be working everywhere <a href="https://mdn.github.io/sw-test/" rel="nofollow noreferrer">https://mdn.github.io/sw-test/</a></p>
  
  <p>This is demo service worker code</p>
  
  <pre><code>self.addEventListener('install', function(event) {
    event.waitUntil(
      caches.open('v1').then(function(cache) {
        return cache.addAll([
          '/sw-test/',
          '/sw-test/index.html',
          '/sw-test/style.css',
          '/sw-test/app.js',
          '/sw-test/image-list.js',
          '/sw-test/star-wars-logo.jpg',
          '/sw-test/gallery/bountyHunters.jpg',
          '/sw-test/gallery/myLittleVader.jpg',
          '/sw-test/gallery/snowTroopers.jpg'
        ]);
      })
    );
  });
  
  self.addEventListener('fetch', function(event) {
    event.respondWith(caches.match(event.request).then(function(response) {
      // caches.match() always resolves
      // but in case of success response will have value
      if (response !== undefined) {
        return response;
      } else {
        return fetch(event.request).then(function (response) {
          // response may be used only once
          // we need to save clone to put one copy in cache
          // and serve second one
          let responseClone = response.clone();
  
          caches.open('v1').then(function (cache) {
            cache.put(event.request, responseClone);
          });
          return response;
        }).catch(function () {
          return caches.match('/sw-test/gallery/myLittleVader.jpg');
        });
      }
    }));
  });
  </code></pre>
  
  <p>When I open developer console, I get this error:
  <code>"Registration failed with TypeError: ServiceWorker script at https://mdn.github.io/sw-test/sw.js for scope https://mdn.github.io/sw-test/ encountered an error during installation."</code></p>
  
  <p>In the same time, this same code works without errors in Chrome.</p>
  
  <p>In <code>about:config</code> setting <code>dom.serviceWorkers.enabled</code> is set to <code>true</code>.
  I have also disabled all plugins and it still isn't working. </p>
  
  <p>What could be the problem?</p>