TiddlyWiki saves (and exports) stopped working with the Firefox upgrade to 15. After trying a few changes to prefs.js, I found the post Will Firefox 16 support TiddlyWiki on the Mozilla site. It had a link to a Firefox extension called Tiddly Wiki 5, which fixed the problems I had with saving and the export plugin.
https://github.com/Jermolene/TiddlyWiki5