Technical Limitation

Firefox WebExtensions are not as powerful as previous legacy extensions, which could use Firefox APIs directly to add new custom search engines. With WebExtension we have to fallback to using another API that is also available to webpages: window.external.AddSearchProvider (See also the MDN article Adding search engines from web pages). This API requires that you use a http: or https: URL, and because we also can't locally host a webserver with the WebExtensions API, therefore the search engine XML file has to be uploaded somewhere. We use file.io because it's free and easy to use without additional permissions.