"Recent updates to Firefox and Chrome include tighter default security settings that can interfere with the operation of TiddlyWiki. The main cause is restrictions on the ability of TiddlyWiki running from a file:// URI to obtain permission to access local files as well as files stored remotely on other servers.

"Previous versions of Firefox asked the user for permission to access the local file system, both for reading and writing files, with an option to retain and re-use that setting whenever the associated TiddlyWiki document is loaded into the browser. Unfortunately, as of version 15.0, FireFox no longer allows requests to access the local file system, and produces an error instead (note: existing TiddlyWiki documents that were already approved for local file access in a prior version of Firefox will continue to work as before)."

This is an issue for users of ACP who use Firefox (v15 and greater) and expect to edit item/tiddlers. "Fortunately there is a fix for this problem: Jeremy Ruston [creator of TiddlyWiki] has written a small, easy-to-install FireFox extension called TiddlyFox that provides replacements for the necessary file I/O functions, allowing new TiddlyWiki documents to be saved locally."

[This from a note from Eric Shulman posted at http://tiddlywiki.com/ -- look for "TiddlyWiki Browser Compatibility" on 16 Sep 2012.]