title: JavaScript/Firefox: "Failed to register/update ServiceWorker", when nothing called ServiceWorker is in use
body: <p>Here's a weird one.</p>
  
  <p>I'm developing a Django/Bootstrap 3 web application, and recently I've started receiving this error in the JavaScript console:</p>
  
  <pre><code>Failed to register/update a ServiceWorker for scope ‘http://localhost:8000/’: Load failed with status 404 for script ‘http://localhost:8000/service-worker.js’
  </code></pre>
  
  <p>The funny thing is that I'm not actually attempting to use anything called ServiceWorker or service-worker.js and never have. I don't know what it's supposed to do. I do develop many other websites on localhost:8000 though, so I'm guessing it's some kind of left-overs from a different project.</p>
  
  <p>The error does NOT show up when I try browsing in private mode, further implying that this is some kind of browser memory.</p>
  
  <p>Still, I've reloaded with Ctrl-Shift-R and Shift-F5, and cleared all cookies for <code>localhost</code>. I've searched both my own code, as well as the entire Python environment for the strings <code>service-worker.js</code> and <code>ServiceWorker</code> but found nothing.</p>
  
  <p>I'd really appreciate help with two things:</p>
  
  <ol>
  <li>Learn what can cause this sort of thing.</li>
  <li>Get rid of the error itself.</li>
  </ol>
  
  <p>I'm using Firefox 64, Django 2.0.10 (with Django Debug Toolbar 1.9.1) with Bootstrap 3.3.7 on Ubuntu 18.04.</p>
  
  <p>(Not even sure how to tag this thing, because I don't know what I'm dealing with.)</p>