User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:41.0) Gecko/20100101 Firefox/41.0 Build ID: 20150929144111 Steps to reproduce: Firefox 41.0.1 is installed on x64 machine running Windows 7 64-bit OS. browser.warnOnQuit is set to default value of true. browser.tabs.warnOnClose is set to default value of true. I click the red X close button in window caption bar. Actual results: Firefox closes immediately without any warning. All open tabs states are retained. The next time Firefox is started, all tabs are exactly as before the prior close, as expected. Expected results: With the settings shown, I would expect Firefox to display a warning dialog, along the line of: [-------------------------------------------------------------------------- X] Warning, you are about to close Firefox. Do you really want to close Firefox? YES - Close Firefox NO - Cancel closing Firefox [----------------------------------------------------------------------------] And clicking YES button would close Firefox, while clicking NO button would return to the browser state immediately prior to the click of the red X close button. Clicking the warning dialog's own X button would perform the same action as clicking the NO dialog button. The [-----] are not to be taken literally.. they are representations of a dialog box's border. This does not appear to be a duplicate of any bug reported in the 41 branch. browser.warnOnQuit appears to be ignored in the 'true' state.