how do you disable the 'confirm close tabs' dialog in Firefox

I have Firefox 16 and really don't want the "confirm close tabs" prompt when I close multiple tabs. I want to close as many tabs as I want and not be asked about it.

I found the property in about:config that sounds like the right one, "browser.tabs.warnOnClose" and set that to false but it didn't help. Can some one tell me how to stop that?