All apps submitted to the Firefox marketplace must be accompanied by a manifest.webapp file that describes the app, its location, it’s icons and permissions (full details). This file must be served from a server with an HTTP Content-Type header of application/x-web-app-manifest+json. This they claim is for security reasons, but makes the process of submitting an app more cumbersome, especially as there are very few instructions on how to do it.

I am using  Tomcat 7 on Linux hosted on Amazon Web Services and to be able to server  the manifest file with this header is was necessary to define in the web.xml deployment descriptor configuration file a new mime type.

    <mime-mapping>  
        <extension>webapp</extension>  
        <mime-type>application/x-web-app-manifest+json;encoding=UTF-8</mime-type>  
    </mime-mapping>

Once the app is packaged into a WAR file and deployed to the server the manifest.webapp file can be validated using the Manifest Validator via a link provide by Firefox.

This, however, does not mean that the manifest file is OK as I found out when I submitted my app to the marketplace.

Some useful links:

Mozilla’s FQA regarding the App manifest.