Firefox is one of many open source products, along with the Linux operating system, the Apache web server, and the OpenOffice Suite. Open source products are products for which the source code is available for users to examine and, if they wish, to modify. This model promotes software reliability and quality via a worldwide community of developers who can submit their own improvements to the code or simply patch it for their specific needs. The open source model differs radically from the "closed source" or "proprietary" model used by Microsoft, Adobe, and most other software manufacturers, who will do almost anything to prevent outsiders from viewing their products' source code.
Open source software is not freeware (although most open source products, including Firefox, are free), nor does it just mean that users can have access to the source code. The license for a true open source product must provide for such things as
Open source products are often released under the General Public License (GNU) or other similar licenses. Firefox is released under the Mozilla Public License, but if you're interested in finding out more, you can find a selection of open source licenses at the Open Source Initiative website (www.opensource.org).
Most open source products have a devoted community of hundreds and even thousands of developers who are deeply interested in making the products stronger and better. As a result, open source software tends to be more robust: as soon as bugs or security holes are found (all software has bugs), the community of programmers usually discusses the problem, and typically several people will develop and release patches to the source code.
According to the Open Source Initiative, open source is good for consumers and businesses because it lets everyone review, distribute, and modify the code for software, which results in a faster evolutionary cycle. You don't have to wait for [insert the name of any large software company here] to identify problems and issue patches or updates for a product; you can usually get a programmer to develop a custom correction or modification right away. In addition, because the source code is available, you're never held hostage by a company's pricing, its desire to release new versions, or its continued existence. The people who use the products can adapt, patch, or even enhance them to meet their own requirements.