Please try the steps in this support article: Change the program used to open email links.

Is Outlook listed and selectable?

I previously read the support article and followed the steps. My problem is once I get to the final step, Outlook is not a choice and I am not able to find it when I browse. No doubt my problem is my lack of familiarity with Windows 10.

Hmm, this is for Microsoft Office Outlook? Normally you don't need to browse for Outlook, Firefox finds it in the Windows Registry as a registered handler for mailto links.

If you migrated your Firefox settings from an earlier version of Office, possibly there could be some settings confusion in one of Firefox's settings files. Unfortunately, the file is very hard to edit, so we generally suggest renaming it and letting Firefox generate a new one. Here's how:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup, then rename mimeTypes.rdf to something like mimeTypes.old. (In case this doesn't help, you can reverse the process later.)

Start Firefox back up again and it should extract default application settings from the registry. Then try the mailto handler setting again. Is Outlook there this time?

Excellent step-by-step instructions but they did not solve the problem. "Mail to" still offers only "use TWINUI"

Your drop-down only shows TWINUI, and a few webmail services?

Does Edge or IE use Outlook to share links? If not, perhaps there are instructions for how to fix that which will also fix it for Firefox.

This is what mine looks like. Press the box to the far right for options.