From MozillaZine Knowledge Base

You can make Firefox launch your webmail (GMail, Yahoo, Hotmail, Netscape, Opera, Mail.com, Horde, SquirrelMail) when you click on an e-mail (mailto:) link by using the WebMailCompose extension. Note that this extension will not change your computer's default mail program, nor will it work for any e-mail links clicked from applications other than Firefox.

For information on using Thunderbird/Mozilla Mail to read your webmail, see this article.