We are working with Firefox Portable ESR 78.6.0 on windows 10 OS We need to detect the close event to override it, or disable it when the user try to close the browser from the taskbar. With an older version of Firefox Portable ESR (52.2.0), we used the following code to detect browser closure: var observerService = Components.classes ["@ mozilla.org/observer-service;1"OUND.getService(Components.interfaces.nsIObserverService); observerService.addObserver (this, "quit-application-granted", false); observerService.addObserver (this, "quit-application-requested", false); observerService.addObserver (this, "browser-lastwindow-close-requested", false); This code is no longer compatible with the version 78.6.0. When the user close the browser from the taskbar we need to check: if (user can close) close browser else do not close the browser So, is there a way or an add-on to install to disable the closing of the browser from the taskbar? Or if possible how can we override the browser's code to prevent the user's click on "close window" ?