This Add-On allows you to install browser extensions straight from GitHub Repositories, the only requirement in the current version is that the extension files are placed in the root of the repo. If that is the case (ie, a install.rdf file is found there), a new button will be added in the right-bottom side of the page as shown in the next screenshot.

Please note the installation is performed in the background, there won't be a confirmation dialog, you'll just get a notification indicating whether the installation succeed or failed.

AMO Listing Page: https://addons.mozilla.org/addon/github-extension-installer/