You could create a script that launches mozilla firefox. I'm sure that you can pass it a URL to open upon start. You would have to create this script and then add it as a New Computing App. The script itself would be very simple, but adding it to the computing apps would take a little effort. There is a page on the wiki on how to add a computing app.
As far as I know there is no way to add some type of widget or shortcut to the main interface. You will have to take the additional step of opening the computing apps menu and then selecting which application you would like to start.