A lot of built-in about: protocol pages like about:addons and about:preferences open in a new tab and it might not be possible to prevent tabs opening completely. For links that specify a target window there is a setting in "Options/Preferences > General". You can look at the Classic Theme Restorer extension to open Options/Preferences in a standalone window. *General UI (1): Open options page (about:preferences) in a window (v2) *https://addons.mozilla.org/firefox/addon/classicthemerestorer/