Although I'm unsure if all these existed in the 10.04 version of Ubuntu, I'm putting this answer here in case it does and also so it can be used for newer releases. You can use upstart to start a GUI application when a user logs in and have it require that networking is up first.
Run a GUI application
Run when a user logs in
This makes use of D-Bus Service Activation.
Add "UpstartJob=true" to file "/usr/share/dbus-1/system-services/org.freedesktop.ConsoleKit.service".
Create a job configuration file corresponding to the D-Bus service, say /etc/init/user-login.conf :
start on dbus-activation org.freedesktop.ConsoleKit
exec /usr/sbin/console-kit-daemon --no-daemon
Ensure that the D-Bus daemon ("dbus-daemon") is started with the --activation=upstart option (see /etc/init/dbus.conf).
Now, when a user logs in, D-Bus will emit the dbus-activation event, specifying the D-Bus service started. You can now create other jobs that
start on user-login.
Depend on another service
start on (started networking and user-login)
By combining these three things in an upstart job, you can start Firefox when a user logs in and the network is started (although you may want to specify which interface is up)