nsIPluginHost
then using nsihandlerservice it gets the handler info.
... let pluginhost = cc["@mozilla.org/plugin/host;1"].getservice(ci.nsipluginhost); let handlerservice = cc['@mozilla.org/uriloader/handler-service;1'].getservice(ci.nsihandlerservice); let mimeservice = cc['@mozilla.org/mime;1'].getservice(ci.nsimimeservice); let plugintags = pluginhost.getplugintags(); for (let i = 0; i < plugintags.length; ++i) { let plugintag = plugintags[i]; let mimetypes = plugintag.getmimetypes(); console.warn('plugintag:', plugintag.name, 'mimetypes:', mimetypes); // go through all the mime types and get the handler service for (let j = 0; j < mimetypes.length; ++j) { let type = mimetypes[j]; let wrappedhandlerinfo = mimeservice.getfromtypeandextension(type, null); ...

nsIExternalProtocolService
specifically, this looks to see whether there are any known possible application handlers in either the nsihandlerservice datastore or registered with the os.