Some webpages contain email links. If you right-click on the link in most Web browsers, a menu appears that lets you copy the email address to the clipboard (first screenshot). You can then paste the address into the To field of a new email message.
In recent versions of Mozilla's Firefox browser, you can also left-click on the link and get some action. If Mozilla's Thunderbird is your default mail program, a Thunderbird 'compose' window may appear with the To field automatically filled in. This article explains how you can get the same automatic result under Linux with the excellent open-source mail programs Sylpheed and Claws Mail. The method also works with Iceweasel, which is the rebranded Firefox packaged with Debian GNU/Linux.