Released versions can be downloaded from https://addons.mozilla.org/firefox/addon/new-tab-tools/ Beta versions are sometimes available from https://addons.mozilla.org/firefox/addon/new-tab-tools/versions/beta
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
Please send a pull request. My translation website is not currently compatible with WebExtensions.