How do I programmatically download a file created on the file with a Firefox WebExtension?

I am trying to port a Chrome Extension that programmatically creates and downloads a file to a Firefox WebExtension, using Firefox 45.0.1.

This is the Javascript code:

  text = '{"greeting":"Hello, World!"}';
  var a = document.createElement('a');
  var file = new Blob([text], {type: 'text/json'});
  a.href = URL.createObjectURL(file);
  a.download = 'hello.world';   // Filename  
  a.click();                    // Trigger download 

All lines seem to execute fine, but no file is downloaded (I put a console.log() after the a.click()).

As of now there is no chrome.downloads API in Firefox WebExtensions.

Is there any incompatibility with Firefox in the code above? Is there any other alternative to programmatically download a file using a Firefox WebExtension?