when I select Gmail in the popup window, it does work and I can send an email.

It sounds like it is working? Just tick "Always use this application" to avoid the popup.

Choosing Firefox doesn't make sense since it already open and is not an email client.

Note that if you select to open in Firefox and would tick 'Always use...', then this will cause Firefox to open new tabs or windows endlessly. You should never delect Firefox in such a case because Firefox wouldn't show this download pop-up if it could handle this file internally.

• https://support.mozilla.org/en-US/kb/firefox-repeatedly-opens-empty-tabs-or-windows

Looks like I've gotten myself a little confused.

Having thought more about what I said previously, I want to use Yahoo to send emails as before--not Gmail.

How do I get the yahoo.com email when going to File >> Email link?

This was always automatic, never a popup window asking me to choose. I understand clicking the check box to always use.

Something changed in Firefox and I don't know what. Maybe an upgrade.

What I meant to say was how to get Yahoo email working again.

In the past, I would go to File >> Email Link. This allowed me to send a Yahoo email. Now, I only see Gmail.

A popup window now appears and there is no Yahoo email to choose from. I understand about checking the box for Always use.

You need to install Yahoo as a "mailto" handler but this needs to be supported by the site. You could try installing the extension Yahoo Mail - mailto and Email Link Fix.

See also:

• /questions/1418668 Yahoo Mail is not an option in mailto settings

You can edit handlers.json and expand the mailto section and add a rule for Yahoo aafter the existing rule for Gmail. Make sure you backup your current handlers.json.

"schemes":{"mailto":{"stubEntry":true,"handlers":[null,{"name":"Gmail","uriTemplate":"https://mail.google.com/mail/?extsrc=mailto&url=%s"},
{"name":"Yahoo! Mail","uriTemplate":"https://compose.mail.yahoo.com/?to=%s"}]}}

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data