For an installable add-on package, click Downloads in the top right corner of this page and download the .xpi file. Save it to disk and drag it into the Firefox window to install. You don't need to restart the browser, thanks to the magic of Jetpack.
UglifyJS. The license for UglifyJS can be found in the COPYING file.
gentleface.com for the icons next-icon-black.png and next-icon-white.png. These icons are licensed under a Creative Commons Attribution-NonCommercial license.
ubuwaits for the CSS3 buttons. The license for the buttons can be found in the COPYING file.