Sorry, but I have no idea. Like I said: it's probably caused by a Firefox or Flash update.
I'm not actively maintaining this project anymore, since I think there are better alternatives to Flash available (that also work on mobile devices). Have a look at a HTML5 based uploader like dropzonejs.com or https://github.com/blueimp/jQuery-File-Upload
Integration of Dropzone with Dropbox isn't very complicated: http://linqed.eu/2015/11/28/dropzone-js-in-xpages-it-doesnt-get-easier-than-this/ and