How to reference a file in the data directory of a Firefox plugin?
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I'm working on a Firefox plugin and I need to inject a javascript in a page from a contentscript. In my Chrome plugin I have done the following:

    this.initializeJplayerSupport = function() {
      var script = document.createElement('script');
      script.setAttribute('type', 'application/javascript');
      script.setAttribute('src', chrome.extension.getURL('js/custom-jplayer.js'));
      document.head.appendChild(script);
    }

The file is in my data directory. How can I reference a js file in a firefox plugin contentscript (where I have used `chrome.extension.getURL()` for Chrome)?