Let's assume that you have a directory called /etc/ppp. In that directory, you should have several scripts, including one called ip-up. If you read that script, you will see that ip-up will look for a script called ip-up.local, and run it if it exists and is executable. If you do not have ip-up.local, create it and add the command lines (one per line) for the apps you want to start when you make your internet connection.
which would be executed by ip-up to start mozilla when the connection is made.