There seems to be no simple way to add an email provider not recognised by the browser. You could look at the thread linked below. https://support.mozilla.org/en-US/questions/1305068

navigator.registerProtocolHandler('mailto','https://compose.mail.yahoo.com/?to=%s','Yahoo! Mail');

For Yahoo you can try this command. navigator.registerProtocolHandler('mailto','https://compose.mail.yahoo.com/?to=%s','Yahoo! Mail'); More: *https://ladedu.com/these-are-the-direct-links-to-composing-emails-in-web-based-email-services/ *https://developer.mozilla.org/en-US/docs/Web/API/Navigator/registerProtocolHandler

Here are a few options: '''''Option 1: Install a Yahoo! add-on''''' Yahoo has several add-ons and I don't know which one has the best balance of useful versus annoying features. https://addons.mozilla.org/firefox/user/1947/ '''''Option 2: Run a Browser Console Script''''' One way to add a new mailto destination, also called a URI handler, without installing an add-on, is to run a script in Firefox's Browser Console, one of the developer tools. This requires a settings change to switch the console from read only to read/write. Because scripts run in the console can use interfaces that are totally illegal for web pages to use, you definitely need to be very careful about what scripts you run so that you don't infect your browser. I posted the script here: https://gist.github.com/jscher2000/8a9112b1c03af77f6b3b63b1296bd864 '''''Option 3: Manual Settings File Edit''''' The "mailto" handling options are stored in a file named '''handlers.json''' in your currently active Firefox profile folder. This file is a huge continuous line of text, so it's a bit annoying to edit. If you want to try it: '''(1) Open your profile folder''' You can open your current Firefox settings (AKA Firefox profile) folder using either * "3-bar" menu button > Help > More Troubleshooting Information * (menu bar) Help > More Troubleshooting Information * type or paste '''about:support''' in the address bar and press Enter In the first table on the page, find the '''Profile Folder''' row and click the "Open Folder" button. This should launch Windows' File Explorer listing the contents of your current profile folder. Without closing that window, switch back to Firefox. '''(2) Exit Firefox''' Use menu > Exit and pause for 5-15 seconds while Firefox cleans up and closes files in the profile folder. '''(3) Make a backup copy of handlers.json''' Right-click the handlers.json file and click Copy. Then right-click a blank area of the window and click Paste. Windows should add a file named along these lines: handlers - Copy.json '''(4) Edit handlers.json''' Right-click handlers.json and choose either Edit or Open With. The goal is to use a simple text editor and not a word processor that might corrupt the file with formatting codes. WordPad might work, or you can use a third party editor like Notepad++ or Visual Studio Code (but these are hard to justify if you don't plan to edit code files). Look for this text: {"name":"Gmail","uriTemplate":"https://mail.google.com/mail/?extsrc=mailto&url=%s"} After that, add a comma and then {"name":"Yahoo! Mail","uriTemplate":"https://compose.mail.yahoo.com/?to=%s"} Save the change, making sure to keep a plain text format if your editor suggests changing the format. ----- '''''After any of these changes:''''' Yahoo! Mail should now be an option on the Settings page, in the Applications box for the mailto protocol. The steps in this article will get you to that setting: [[Change the program used to open email links]]. Success?