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

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

pref("general.config.filename", "mozilla.cfg");
pref("general.config.obscure_value", 0); // use this to disable the byte-shift

See:

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

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

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

See also:

• https://mike.kaply.com/2012/03/16/customizing-firefox-autoconfig-files/
• https://mike.kaply.com/2012/03/22/customizing-firefox-advanced-autoconfig-files/

Autoconfig files are JavaScript files that have full access to all components of Firefox.

how to apply this to Group policy

Maybe this extension will help (created by the above mentioned Mike Kaply).

• https://addons.mozilla.org/en-US/firefox/addon/cck/

sorry this tools didn't solve the problem I want to add Pac file setting for Firefox to all user in my company. what's the relation by this plugins?

Regards Ahmed

You can try to contact Mike Kaply to see if he has a suggestion on how to proceed on this.