I see some apps such as Thunderbird and Firefox add a "force quit" to their dock menus.
where is this menu configuration stored?
if it's not a simple text file change, how can I add this to other applications?
update: Thanks for the hints about pressing alt/option to get the force quit menu item. Note that option-Menu doesn't do this. You have to bring up the context menu by right-clicking or control clicking, and then pressing the alt/option key. You will see the "quit" menu item change to "force quit".