We should see if there are things that can be done to improve the perception of speed when the user is about to close a tab. Eg. - can we prepare the tab that will be displayed next by removing any throttling of JS there? - if the next tab is a lazy browser after a session restore, should we start loading the page in it? - can we reduce the priority of the current tab's JS so that we have more resources available for the next tab that will be shown? - how badly would we break the web if we fired the onbeforeunload even on mousedown on the close button rather than on click?