Mozilla Tri-Pack is a program that installs all three of Mozilla's main programs. It includes Mozilla Firefox, Mozilla Sunbird, and Mozilla Thunderbird. Firefox is a Web browser, Thunderbird is an e-mail client, and Sunbird is a calendar program. Mozilla is a much more secure platform than Microsoft's products.
An example of this is that Outlook and Internet Explorer are prone to viruses. The Mozilla Products are not. This also makes it so that you do not have to download all of the programs separately. They are all compressed in one convenient file.