Ubuntu – How do run a program at startup

13.04startup

I'm trying to make Firefox run when users log on (and go to a specific page if possible), but I'm not sure how.

Using Ubuntu 13.04, I can enter the Startup Applications Preferences (by pressing Alt+F2 and typing gnome-session-properties). However, after clicking Add when browsing for a command, I'm a bit stumped.

Where would I need to point to in order to start Firefox? I did try putting just http://www.mywebsite.com/, but that did nothing. Thanks.

/usr/bin/firefox should do it. You can even specify a webpage to open:
/usr/bin/firefox http://www.google.com/

If you are ever stumped as to where to find a specific command, run which <command> in a terminal to find out where it is.