The colour of links in emails has to be set up in the email client. Are the links not printed in a different colour? You can choose what colour the links are if you are using Thunderbird and set that up in a userContent.css file. The colour of links in emails has to be set up in the email client.

The color of the links are already set up in Yahoo mail for the purpose of accessing the address, but not appearing in FireFox which I use. I am assuming FireFox does use that function.

You can set the colour of links in userContent.css with Firefox.

Just above the textarea that you use to enter the message text on the "Compose New Mail page" there is usually a button bar with buttons to add text formatting like Bold and Italic.
That toolbar may also have a button to turn a text link into a clickable hyperlink (look for a chain like button).
You can select the text and click that button to turn the link into a clickable hyperlink.
If you can't find the button then hover them all to check the tooltip of each (e.g. Insert hyperlink).

• Make Link - https://addons.mozilla.org/firefox/addon/make-link/