According to Mozilla, App Validator is a tool that is built to test those apps that are ready for submission to Firefox Marketplace. Apps submitted to Marketplace are automatically tested using App Validator, which is the first step of the reviewing process.
App Validator checks various points, like syntax of the manifest file, whether the launch_path exists or not, if there are there any CSP violations, or if the app contains any JS syntax errors. There are various other things that App Validator checks to ensure the security of both the app and the user.
Note
App Validator is available both online at https://marketplace.firefox.com/developers/validator and offline at https://github.com/mozilla/app-validator.
To use the App Validator offline, first install it using the setup instructions that are provided, and after installing the validator, open Terminal (MAC or Linux users), or command prompt (Windows users), and type the following code:
app-validator /path/to/your...