It is not checked in the Log-in page.
So, it's not "checked" --- but it
is there then?
Because the checkmark only toggles whether the app will be visible or hidden. If it's checkmarked, it will be hidden. If it's not checkmarked, it will be visible. (as in your situation now)
But so long as it even
exists in that list, it will be launched.
So select it and click the the minus sign in the
[+|-] widget at the bottom, to remove it from the Login Items list.
Too bad, i just cooked up a wicked cool terminal command, but we probably don't need it...
grep -i Firefox {,/System,~}/Library/{Launch{Agents,Daemons}/,\
Preferences/com.apple.login,StartupItems/}* /etc/mach_init*/* 2>/dev/null
