There is only one file type that uses the .WEBAPP extension:
Firefox Web App Manifest File
Web app manifest file used by Firefox OS apps to identify important details of an app; contains information in JSON format about an app, which includes the name, developer, launch path, description, and icon(s); also contains a list of web APIs that the app requires.
The list of web APIs contained within the WEBAPP file allows you to make decisions about apps before installing them. WEBAPP files are necessary in order to publish an app to the Firefox Marketplace.
NOTE: You can scan your WEBAPP manifest file for correct formatting and appropriate values with the Mozilla Web App Validator.
manifest.webapp - The naming convention used for web app manifest files used by the Firefox OS app. Placed in the root of the app directory.
|Mozilla Web App Validator — Discontinued||Free|
Updated: May 29, 2018