When TiddlyWiki goes to save a file, in Firefox, you get the following dialog

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Internet Security

A script from "file://" is requesting enhanced abilities that are UNSAFE and could be used to compromise your machine or data

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You'll often read doc about updating prefs.js with something that resembles:

user_pref("capability.principal.codebase.p0.granted","UniversalXPConnect  UniversalBrowserRead");
user_pref("capability.principal.codebase.p0.id", "file://"); 

This advice is almost correct, but you you specify only "file://" you will still get the message. You must specify the full path name of the file. Instead of "file://", give it something like:

user_pref("capability.principal.codebase.p1.granted", "UniversalXPConnect");
user_pref("capability.principal.codebase.p1.id", "file:///C:/Documents and Settings/kevin/Desktop/TwoNote/ScrapBook.html");
user_pref("capability.principal.codebase.p1.subjectName", "");

Firefox no longer accepts that "file://" is good for "file:///blah-blah/blah.html