Internet Explorer, abbreviated IE or MSIE is a proprietary web  browser made by Microsoft and currently available as part of Microsoft Windows. Internet Explorer is by far the most widely used web browser, although since 2004 it has lost some usage share to other browsers.

Internet Explorer is an integrated component of newer versions of Microsoft Windows. It is available as a separate, free-of-charge product for many older versions of the operating system. It has been shipped as the default browser in all versions of Microsoft Windows since Windows 95 OSR2. The last major upgrade to Internet Explorer, however, was only offered for Windows XP Service Pack 2. Initially Microsoft planned to release Internet Explorer 7 with the next major version of Windows (Windows Vista), but the company has recently announced that a testing version of Internet Explorer 7 will be available for Windows XP SP2 users by "the summer of 2005" (i.e. mid-2005).

Mozilla Firefox (originally known as "Phoenix" and  briefly as "Mozilla Firebird") is a free, cross-platform, graphical web browser developed by the Mozilla Foundation and hundreds of volunteers. Before its 1.0 release on November 9, 2004, Firefox had already gained acclaim from numerous media outlets, including Forbes and the Wall Street Journal. With over 25 million downloads in  the 99 days after its release, Firefox became one of the most used free and open source applications, especially among home users. On July 26, 2005, Firefox had its 75 millionth download, less than 9 months after the release of version 1.0.

With Firefox, the Mozilla Foundation aims to develop a lightweight, fast, intuitive, and highly extensible standalone browser based on the Navigator component of the Mozilla Application Suite. Firefox has become the foundation's main development focus (along with its Thunderbird email client), and has replaced the Mozilla Suite as their official main software release.

Firefox includes an integrated pop-up blocker, tabbed browsing, live bookmarks, support for open standards, and an extension mechanism for adding functionality.  Although other browsers have introduced these features, Firefox  became the first such browser to achieve wide adoption.