Dr. Body Narrow is a Firefox add-on, helps you to read text on wide screens. It Provides various options to limit current page width (so it won't occupy 100% of window).

Just click Dr.Body Narrow toolbar icon and current page will change its width, making it comfortable to read too-wide-texts. You will be able to choose one of 600, 760, 960, 1240, 1560, 1880 pixels on your choice, which one is more comfortable for you.
Also Dr.Body Narrow allows you to stick page to the left or right side of the window, or center it - as you like.

Requirements:
* Firefox

The license of this software is Free, you can free download and free use this browser plug-ins software.