how to kill a session by closing window or tab clicking on X?

hi. I'm sorry for posting this more javascript then php question, but it's somehow php related. here is the issue: very often people close the window/tab without logging out. I need solution how to "recognize" when [x] is clicked (or File >> Close) and kill the session before the window/tab is closed. few years ago, before firefox and tabs, I solved this by javascript and onClose() as a part of body tag. now, it doesn't work anymore. any suggestion/opinion/experience? thanks afan