jba5, a list of Firefox keyboard shortcuts can be found here: https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly (see the "Windows & Tabs" section for the various closure combinations) Also, if you type or paste "about:keyboard" into the address bar and press enter, there might be options for changing or removing any combinations that you find yourself pressing by accident. For more information see here: https://support.mozilla.org/en-US/kb/customize-keyboard-shortcuts-firefox