
you may want to check http://firefoxprivileges.tiddlyspot.com.
When this prompt appears as a result of loading a file from your hard drive, you may not be aware of the exact meaning of the 'Remember this decision' check box. While you probably mean "I trust this file and I don't want you to annoy me anymore", Firefox understands "I trust every html file loaded from the hard drive". This broad trust is not a good idea from the security point of view.
This tool allows to easily cancel that effects of "remember this", and tell Firefox that you only trust some specific files (per-file permissions). Before hacking it, I used to have to mess with the user.js file, much less convenient.
The tool is implemented as a TiddlyWiki plugin. If you are not familiar with TiddlyWikis (super cool wikis in a single file), this is a good chance to get to know them.
Edit:
Some strings to make this post googleable for the right people:
UniversalXPConnect UniversalBrowserRead UniversalBrowserWrite UniversalFileRead CapabilityPreferencesAccess UniversalPreferencesRead UniversalPreferencesWrite
netscape.security.PrivilegeManager.enablePrivilege signed.applets.codebase_principal_support