It should appear a second after the page seems to have finished loading, so that it hijacks the input if the user is navigating with the keyboard or typing something into a form.
If you log out, and then (in Firefox) close the window... oh wait, you're not really logged out. Try again.
If you log out in Firefox, wait until the window is quiet, then delete the cookie... LinkedIn creates a new cookie... which still sometimes logs you back in but not as often.
Opening a new window, deleting the LinkedIn window (fuck watever it's doing), and THEN delete the cookie in the NEW window... then it seems to stay gone at least as of today.
Man, I need to unplug.