Mozilla Thunderbird is a stand-alone mail client (i.e. no browser+mail client combo) that is intended on users who want to stick with their browsers (such as IE), but want a new mail client to replace the default (such as Outlook Express), and to take advantage of all the extra add ons and benefits that it has to offer.
Please note: Once payment is made you need to send a private message to wicked at the forum, stating your username and the email address you used to pay. If you do not do this your upgrade will be delayed, the upgrade will be done within 12 hrs.