Too Wide! от Mark Simon

Sites such as Wikipedia insist on using the full width of the screen. For many of us, this means it renders the page much too wide to be comfortably readable.

This add-on adds a contextual (right-click) menu which allows you to toggle the width of the current page.

By default, the new width will be 960px. You can adjust this in preferences.

Although designed to work with Wikipedia and its cousins, it will also work with many other pages designed a long time ago, before wide screens became fashionable.