I am not sure that may be done. Some addons may allow some sort of flaky use of GPO See also

• A related thread: Managing extensions in enterprise /questions/1163093
• MDN: Deploying Firefox in an enterprise environment
  https://developer.mozilla.org/Firefox/Enterprise_deployment
• EWG: https://www.mozilla.org/firefox/organizations/faq/

Note you may be far better using Firefox ESR given all the impending changes likely to be coming through in ordinary Releases in the near future.

Someone else may be able to give you a more comprehensive reply but we mainly try to support ordinary end users rather than answering enterprise deployment questions here.

You can use the mozilla.cfg file in the Firefox program folder to set or lock preferences and run privileged JavaScript code.

A local-settings.js file needs to be placed 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);

The mozilla.cfg file needs to be in the main Firefox program folder.

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

The mozilla.cfg file and possibly local-settings.js need to start with a comment line (//).

See Configuration:

• https://developer.mozilla.org/en-US/Firefox/Enterprise_deployment

See also:

• http://kb.mozillazine.org/Locking_preferences
• http://mike.kaply.com/2012/03/16/customizing-firefox-autoconfig-files/

The "browser.startup.homepage" pref is a complex pref and requires a different approach to set a default value.

• https://mike.kaply.com/2012/03/15/customizing-firefox-default-preference-files/
• https://mike.kaply.com/2012/08/29/setting-the-default-firefox-homepage-with-autoconfig/