gistfile1.js

//Sets up the fullscreen editor/previewer
if (fullScreenApi.supportsFullScreen) {
  var fsElement = document.getElementById(self.settings.id);

  //...codes

  self.iframe.getElementsByClassName('epiceditor-fullscreen-btn')[0].addEventListener('click',function(){
    if(_getStyle(self.previewer,'display') === 'block'){
      revertBackTo = self.previewer;
    }
    fullScreenApi.requestFullScreen(fsElement);
  });
  fsElement.addEventListener(fullScreenApi.fullScreenEventName,function(){
    console.log('TEST')
    if (fullScreenApi.isFullScreen()) {
      fsBtns.style.visibility = 'hidden';

      //...codes

      self.preview(true);

      var fullscreenLivePreview = self.editor.addEventListener('keyup',function(){
        self.preview(true);
      });

    }
    else{
      //...codes
    }
  }, true);
}

I guess you have to change this line:
fsElement.addEventListener(fullScreenApi.fullScreenEventName,function(){
to
document.addEventListener(fullScreenApi.fullScreenEventName,function(){

for Firefox to fire the event.