browser.tabs.warnOnOpen is the wrong preference. I've got no idea what the you've set the correct preference to, as you didn't report your bug with Apport (as requested in the bug reporting guidelines - https://wiki.ubuntu.com/MozillaTeam/Bugs)
** Changed in: firefox (Ubuntu) Status: Confirmed => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1064985 Title: Firefox 16. Warning about close multiple tabs is always on To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1064985/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs