You can use following Puppet snippet to set the URL opened by default. Mozilla Firefox and Chromium were rather undocumented and the examples on the Internet were outdated:

# Set Mozilla Firefox homepage
file_line { "firefox-homepage":
    path => "/etc/firefox/syspref.js",
    ensure => present,
    match => '^user_pref\("browser\.startup\.homepage",',
    line => 'user_pref("browser.startup.homepage", "https://www.koodur.com");'
}

# Set Chromium homepage
file { "/etc/chromium-browser/policies/recommended/homepage.json":
  ensure => file,
  mode => 644,
  owner => root,
  group => root,
  content => "{\n  \"RestoreOnStartup\":4, \"RestoreOnStartupURLs\":[\"https://www.koodur.com\"]\n}\n"
}

Note that on your Puppetmaster you might have to install additional module:

puppet module install puppetlabs-stdlib