I have looked a few options to set default homepage options for all users. the options below added to /etc/firefox/prefs.js and /usr/lib/firefox/defaults/pref/prefs.js. Nothing seems to work and there isn't clear documentation that covers the latest deb version pushed out through Firefox's apt repo (non-snap version). I got the impression some release notes these config files are no longer used, which tallies with the fact they don't seem to change anything.
user_pref("browser.startup.homepage", "https://www.test.ac.uk");user_pref("startup.homepage_override_url", "https://www.test.ac.uk");user_pref("startup.homepage_welcome_url", "https://www.test.ac.uk");Because of the odd method Firefox generates pref files, I'm having to resort to a fairly hacky methods to push lines of config into a given users config files: ~/.mozilla/firefox/*.default-release/prefs.js. This isn't ideal as I want to override other options to stop the Firefox welcome page as these are shared machines.