here's a quick batch script to take care of this issue in about 0.2 seconds, you could then deploy it in their login scripts if you see fit. please note that if something has already been configured using preference locking in firefox this will overwrite it.
set loc=no
if exist "C:\Program Files\Mozilla Firefox" set loc=c:\Program Files\Mozilla Firefox
if exist "C:\Program Files (x86)\Mozilla Firefox" set loc=c:\Program Files (x86)\Mozilla Firefox
if "%loc%"=="no" goto done
ECHO pref("general.config.obscure_value", 0); > "%loc%\defaults\pref\local-settings.js"
ECHO pref("general.config.filename", "mozilla.cfg"); >> "%loc%\defaults\pref\local-settings.js"
ECHO // > "%loc%\mozilla.cfg"
ECHO lockPref("signon.rememberSignons",false); >> "%loc%\mozilla.cfg"
:done
pause