You can use the Dr. Body Narrow extension for Firefox. It's more popular, and has more features than the other option. It supports Firefox 3.5+.
About this Add-on:
Dr.Body Narrow helps you to read text on wide screens. It Provides
various options to limit current page width (so it won't occupy 100%
In new versions you are able to choose one of 600, 760, 960, 1240,
1560, 1880 pixels on your choice, which one is more comfortable for
Dr.Body Narrow allows you to stick page to the left or right side
of the window, or center it - as you like.
You can use the Page Width Resizer script for Greasemonkey
(installation instructions). It's very useful for wide screens,
and the functionality of this script is actually very nice. It
supports Firefox 4.0+.
Adds translucent borders to both sides of the page. By
dragging one of the borders, you can change the width of each page
without resizing the window (preserving the page sizes in other tabs).