the issue was opened for v.16 "Firefox 16 asks for confirmation for closing other tabs" and reported as "fixed" but i have the version 20.0.1 and the issue is still not solved. I mention that the checkbox "Options/Tabs/Warn me when closing mutiple tabs" is unchecked and the property browser.tabs.warnOnClose = false but still the browser ask for confirmation when "Close Other tabs" is executed.