Hmm, no, there isn't a built-in setting for that or for other pages that Firefox automatically opens in a new tab like Options, Add-ons, etc. If you pretty much never want a new tab, you could try my "I Hate Tabs" extension that moves any new tab to a new window: https://addons.mozilla.org/firefox/addon/i-hate-tabs-sdi-for-firefox/ Unfortunately, it makes it impossible to use the Customize feature, so if you need to customize frequently, disabling/enabling the extension could be annoying. I really should create a toolbar button for that...