Add handler for Firefox via Browser Console - Yahoo! Mail example

mailto-yahoo-2023.js

/* Add Handler via Firefox Browser Console -- mailto: Yahoo! Mail 

   NOTE: BEFORE YOU CAN RUN THIS SCRIPT, IF YOU HAVEN'T ALREADY:
   * Type or paste about:config into the address bar and press Enter
   * Click the button promising to be careful
   * In the search box type devt and pause while Firefox filters the list
   * If devtools.chrome.enabled is false, double-click it to toggle to true

Paste this entire script into the command line at the bottom of the Browser Console (Windows: Ctrl+Shift+j)
Then press Enter to run the script. A dialog should pop indicating whether the save was successful.
*/

// User-supplied values for the new handler:
var handlerprotocol = 'mailto';
var handlername = 'Yahoo! Mail 2023';
var handlerurl = 'https://compose.mail.yahoo.com/?to=%s';

/* Code to add the new handler, derived from 
https://searchfox.org/mozilla-release/source/browser/components/protocolhandler/WebProtocolHandlerRegistrar.sys.mjs#133
as of 2023-06-18
*/
var handler = Cc["@mozilla.org/uriloader/web-handler-app;1"].createInstance(Ci.nsIWebHandlerApp);
handler.name = handlername;
handler.uriTemplate = handlerurl;
var eps = Cc["@mozilla.org/uriloader/external-protocol-service;1"].getService(Ci.nsIExternalProtocolService);
var handlerInfo = eps.getProtocolHandlerInfo(handlerprotocol);
handlerInfo.possibleApplicationHandlers.appendElement(handler);
handlerInfo.alwaysAskBeforeHandling = true;
var hs = Cc["@mozilla.org/uriloader/handler-service;1"].getService(Ci.nsIHandlerService);
try {
    hs.store(handlerInfo);
    alert('Assuming not a duplicate, it should be saved now. Reload Settings page to see updated list.');
} catch(err) {
    alert('Error during saving: '+err);
}