THERE IS NO BUTTON AT THE BOTTOM TO MANAGE ALIASES

Did you try disabling the extension on the Add-ons page? Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. On the right side, find the "Manage Your Extensions" heading.

Then look for it on the right side. Click the slider switch to enable/disable it.

Can you access Yahoo in another browser, such as Microsoft Edge?

nothing works , why use another browser this is a FF deal.

There MUST be a way to turn this thing off

So you went to this page

https://relay.firefox.com/accounts/profile/

and you found your alias, and you want to either disable it for now or delete it. Do you have these controls: