There is usually a toolbar button in the editor to turn a text link into a clickable hyperlink (look for a chain like button).
You can select the text and click that button.
If you can't find the button then hover them all to check the tooltip of each.

Thanks, but the problem is that the clickable hyperlink does not exist in the toolbar editor. It is not that I can't find it; I am familiar with the icon. The hyperlink button does not show up in Internet Explorer either, but if I hit the spacebar after the web address, the hyperlink appears when I am in IE but this is not the case in Firefox. If you know how to add the hyperlink button to the editor toolbar, please let me know. Perhaps this is an Earthlink Webmail issue, but I contacted them and they were not able to resolve.

Have same problem: IE works, Firefox not. But it is ap dependent -- Ffox works fine in two of my programs, not in two others. IE works in all of them. I have posted lately here: https://support.mozilla.com/en-US/questions/752272.

Thank you JiGong! It is annoying to waffle back and forth between browsers to access different programs, but at least I know it's not just me.