Released versions can be downloaded from https://addons.mozilla.org/firefox/addon/new-tab-tools/
Download from the Chrome store https://chrome.google.com/webstore/detail/new-tab-tools/gfonmeedigdbjpgpeojioadpimflbogh
To get a working version of this repo in your Firefox profile, clone it, then link it into your extensions directory as newtabtools@darktrojan.net and start Firefox.
git clone git://github.com/darktrojan/newtabtools.git
realpath newtabtools > [your profile dir]/extensions/newtabtools@darktrojan.net
To get a working version of this repo in Chrome/Chromium, clone it, then switch to the chrome branch. From the Extensions page, click "Load unpacked extension…".
Please send a pull request. Here's some information that might be helpful (it's for another extension, but mostly applies to New Tab Tools).
The files in webextension/lib are from https://github.com/gildas-lormeau/zip.js.