Legible is a Firefox extension that removes all fixed elements and prevents elements from scrolling to reduce distraction while reading in the browser and increases screen real-estate dedicated to the article/text.
Clicking the toolbar button again brings all hidden elements back.
If you simply want to install the extension, head over to the addons page.
In order to package the extension once you've made changes to it, you must have the Firefox Addon SDK installed. Then you can use the cfx tool in the terminal to package and install the extension by running
cfx xpi
cfx run