There is usually a button bar with buttons to format text like Bold and Italic just above the text area where you compose and edit the message text.
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 link 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).

Different "rich text" editors handle URLs differently. Some may convert them to links immediately, some may convert them when you Save a draft, and some might just leave them.

Usually there is a link button among the formatting buttons that you can use to insert a link. Since I don't have a Talk21 account, I've attached a graphical example of how it works on Yahoo! mail.

Thanks guys. Have used the 'chain' to insert a clickable link. Is this a safety feature of Firefox that this does not happen automatically like it does in IE and Chrome? Thank you again for the super quick response!  :-)

You're welcome

This is likely a limitation of the JavaScript code used by websites to automatically create a hyperlink that works for IE and possibly Google Chrome, but doesn't support Firefox.