Prism, the predecessor to Firefox apps
Firefox apps appear to be based on the premise of Mozilla’s Prism, which is no longer being developed by them. It allowed you to install a web link as an application shortcut on your computer’s OS. Clicking such a shortcut would launch a stripped down version of Firefox (minus its standard GUI), which in turn would go to the URL and load the web-based app (or page) at the address. Chrome has a similar feature built into it.