My work VPN client doesn't work in Firefox, so I hacked something much more specific together to deal with that problem

  #!/bin/sh
  
  if [ "$@" = "/tmp/gpauth.html" ]
  then
    flatpak run --filesystem=/tmp/gpauth.html org.chromium.Chromium /tmp/gpauth.html
  else
    /usr/bin/xdg-open "$@"
  fi