From LQWiki

The first browser wars were an economic competition for mindshare between Netscape's web browser, and Microsoft's Internet Explorer, which occured in the mid-1990s. Other companies' web browsers may also have been involved, but they are largely forgotten now. Microsoft, taking advantage of its monopoly position in the desktop operating system market, bundled Internet Explorer along with Windows, giving it an artifical advantage. Netscape lagged behind as their browser, now part of the Communicator suite, became buggy and bloated. When it became aparent that they would lose the war, Netscape released its source code for the Communicator suite into open source, forming the Mozilla project. However, the Mozilla project determined that Netscape's code had spent too many development cycles as closed source, with too many new features added and too few, overly-hacked bugfixes. The Mozilla project decided to start from scratch. Netscape currently survives as part of AOL, and their current browser is based on Mozilla.

Meanwhile, Microsoft, lulled by their monopoly, allowed Internet Explorer's development to stagnate. This allowed new browsers to develop, creating a new, second browser war. The new browsers that are challenging IE on the Windows desktop include: Mozilla, Mozilla Firefox (a stand-alone spinoff of the Mozilla suite), and Opera.