Sauce Connect Launcher (SCL) is a Firefox add-on that allows users to run a Sauce Connect tunnel directly from Firefox without having to download or install it manually. With SCL, users can easily launch a test by going to Tools > Web Developer > Launch SC and start their tunnel, which can be stopped using the same option. This tool enables cross-browser testing on localhost, making it easier for developers to run manual tests. The add-on requires only a Sauce username and access key to be inputted to establish the tunnel.