I figured that by now most people would have grown accustomed to Facebook's Timeline interface, but my post from a couple months ago about a Firefox add-on that did that same thing. Lo and behold, I found the very same extension.remains one of my most-clicked-on stories. Given the apparent interest in this topic, I searched for a
Install the TimeLineRemove extension for Firefox by clicking the link and then clicking Allow. Then on the next window, click the Install Now button.
The add-on is enabled by default. Head to Facebook or refresh the profile you're currently viewing to see the old, single-column, pre-Timeline design.
Unlike the Chrome version of the extension, the Firefox add-on does not add a button to the browser. To disable the extension and view Facebook's Timeline once again, you will need to head to Firefox's Add-on Manager. On a PC, it's Firefox > Add-ons. On a Mac, it's Tools > Add-ons.