Firefox 15 cannot save TiddlyWiki changes

I just upgraded Firefox to version 15 for Ubuntu 11.04. Everything went smoothly except that TiddlyWiki won’t save changes in Firefox 15. It’s been working well in the old Firefox 6. This is the error message I got while trying to save:

It's not possible to save changes. Possible reasons include:
- your browser doesn't support saving (Firefox, Internet Explorer, Safari and Opera all work if properly configured)
- the pathname to your TiddlyWiki file contains illegal characters
- the TiddlyWiki HTML file has been moved or renamed

After some searches online, I found the solution that works for my case. Locate prefs.js in ~/.mozilla/firefox/<some random text>.default/ folder, then add these two lines to this file:

user_pref("capability.principal.tiddlywiki.granted", "UniversalXPConnect");
user_pref("capability.principal.tiddlywiki.id", "file://<path to tiddlywiki file>");

This change will grant per file permission for the TiddlyWiki file and problem solved.