Firefox
Set default home page
edit /etc/firefox/syspref.js
At the bottom of the file add:
user_pref("browser.startup.homepage", "http://google.com");
Obviously change "http://google.com" to the page of your choice (or not).
This change will not affect existing firefox profiles for existing users. You can reset all users home pages with
sudo sed -i -e '/startup.homepage/d' /home/*/.mozilla/firefox/*/prefs.js
change "/home/*" to "~user_name" for a single user.
sudo sed -i -e '/startup.homepage/d' ~bodhi/.mozilla/firefox/*/prefs.js
Lock home page
You can lock the home page for all users by editing /etc/firefox/syspref.js and using
lockPref("browser.startup.homepage", "http://google.com");
Although again this does not affect existing profiles, you would need to run the sed command again ;)