You can lock the signon.rememberSignons pref to false via the mozilla.cfg file. lockPref("signon.rememberSignons", false);

You canuse a mozilla.cfg file in the Firefox program folder to lock prefs or specify new (default) values.

Place a local-settings.js file in the defaults\pref folder where also the channel-prefs.js file is located to specify using mozilla.cfg.

pref("general.config.filename", "mozilla.cfg");
pref("general.config.obscure_value", 0);

These functions can be used in the mozilla.cfg file:

defaultPref();	// set new default value
pref();		// set pref, allow changes in current session
lockPref();	// lock pref, disallow changes

See:

• http://kb.mozillazine.org/Locking_preferences

• http://mike.kaply.com/2012/03/16/customizing-firefox-autoconfig-files/
• http://mike.kaply.com/2014/01/08/can-firefox-do-this/

I added the local-settings.js and mozilla.cfg files to the C:\Program Files\Mozilla Firefox\defaults\pref directory and the remember passwords option still isn't disabled.

contents of my local-settings.js: pref("general.config.filename", "mozilla.cfg"); pref("general.config.obscure_value", 0);

contents of my mozilla.cfg: lockPref("signon.rememberSignons", false);

The first line of mozilla.cfg never gets read. This line needs to be just two forward slashes.

The contents of the file should look like:

   //
   lockPref("signon.rememberSignons", false);

This file should be in C:\Program Files\Mozilla Firefox\

Only local-settings.js should be in C:\Program Files\Mozilla Firefox\defaults\pref\

If making these changes still does not apply the settings, refer to http://kb.mozillazine.org/Locking_preferences for detailed instructions.