title: createObjectURL blob url not secure in Firefox
body: <p>I'm trying to save downloaded file from secure url (https) with browser but I'm having problems with Firefox.</p>
  
  <p>I use indexedDB to store file in browser memory and when download is finished, I try to save the file in my computer (I'm using a Mac but I think it's not important)</p>
  
  <p>I have this piece of code:</p>
  
  <pre><code>var fileRequest = fileHandle.getFile(); //from indexedDB
  fileRequest.onsuccess = function(event){
  
  {...}
  var file = event.target.result;
  var url = window.URL.createObjectURL(file, {type : fileMimeType, autoRevoke : true});
  
  //I did this with form and not with a href because:
  //https://bugzilla.mozilla.org/show_bug.cgi?id=979227
  
  var form = document.createElement('form');
                          form.action = url;
                          document.body.appendChild(form);
                          form.submit();
  </code></pre>
  
  <p>The browser asks to save this file in Downloads folder and everything seems to work fine BUT it prompts always this message:</p>
  
  <p>"The information entered on this page will be sent through a not secure connection and could be read by third parties. Are you sure you want to send this information?"</p>
  
  <p>If you click 'OK' the file is saved fine, but this security warning is the worst think a user wants to read in a webpage, so the user scares and runaway.</p>
  
  <p>The created url by createObjectURL is a secure url too because is like:
  blob:https//blahblah</p>
  
  <p>This warning doesn't appear in Chrome (using his own filesystem method).</p>
  
  <p>Please I need help :(</p>