Additional information In troubleshooting mode, the icon returns as normal. Yahoo mail in Crome browser, with all extensions active works OK and the send icon is shown as normal.

I have deleted all extensions and in Firefox, I cannot see the send icon in Yahoo mail

Thanks for the suggestions.

As said, Yahoo mail worked normally, showing the send icon in a Crome browser.

Some hours later I went back to Firefox and Yahoo mail and to my surprise the Send icon had re-appeared.

I have re-installed all the extensions deleted previously and again, to my surprise, the Send icon is showing normally.

It remains a mystery, but I'm glad all is good again! Yhanks again for your input.