Center-Pages Browser Extension
If you use a wide screen, you may have encountered websites with long endless lines across the whole width of your screen or websites which align to the very left side of your screen so close to the border that it hurts. For such websites you can use Center Pages. Center-Pages simply adds a margin to the sides, which you can increase/decrease via two buttons. Just have a look at the animated gif above.
You can get Center-Pages for Firefox at https://addons.mozilla.org and for Chrome at https://chrome.google.com/webstore/category/extensions.