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 aremains one of my most-clicked-on stories. Given the apparent interest in this topic, I searched for a Firefox add-on that did that same thing. Lo and behold, I found the very same extension.
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.