Download page - http://www.mozilla.org/products/firefox/

Currently has 20 million users and allows you to import your favorites, settings and other information from Internet Explorer.

Currently offers as standard,

• Popup Blocking
• Tabbed Browsing
• Privacy and Security - set of privacy tools
• Google Search built into toolbar
• Live Bookmarks
• Hassle-Free Downloading
• Has Bookmarks, History, Full Screen, Text Zooming
• Small at only 4.7MB (Windows)
• Comes with developer tools.