The Open-Source Licenses of the Firefox Web Browser

This article provides a straightforward breakdown of the open-source software licenses governing the Mozilla Firefox web browser. It covers the primary license used for the core source code, the historical shift from a tri-license model, the compatibility with other major open-source licenses, and how third-party components and trademarks are handled within the codebase.

The Primary License: Mozilla Public License 2.0

The vast majority of the Firefox web browser's source code is distributed under the Mozilla Public License 2.0 (MPL 2.0).

The MPL 2.0 is categorized as a "weak copyleft" license. It functions as a middle ground between permissive licenses (like the MIT License) and strong copyleft licenses (like the GNU General Public License). Under the MPL 2.0:

  • Modification and Distribution: You are free to use, modify, and distribute the Firefox source code.
  • File-Level Copyleft: If you modify existing MPL-licensed files, you must release those modified files under the MPL 2.0 and make the source code available.
  • Proprietary Integration: You are allowed to combine MPL-licensed code with proprietary files in a larger project. The new, separate files you create do not have to be open-source, provided the original MPL files remain accessible under their original license.

GNU GPL and LGPL Compatibility

Historically, Mozilla distributed Firefox under a complex "tri-license" scheme, allowing developers to choose between the MPL 1.1, the GNU General Public License (GPL) 2.0, or the GNU Lesser General Public License (LGPL) 2.1.

When Mozilla updated its licensing to MPL 2.0, the tri-license was no longer necessary. The MPL 2.0 was written to be natively compatible with both the GNU GPL (version 2.0 or later) and the GNU LGPL (version 2.1 or later). This means the Firefox source code can still be legally copied, modified, and redistributed under the terms of the GPL or LGPL if a developer needs to combine Firefox code with a strictly GPL-licensed project.

Third-Party Open-Source Licenses

Like most modern software, Firefox incorporates various third-party libraries and components to handle tasks such as media playback, graphics rendering, and data compression. Consequently, specific portions of the Firefox source tree are governed by other open-source licenses. The most common among these include:

  • The Apache License 2.0
  • The MIT License
  • The BSD License (2-Clause and 3-Clause variants)
  • The ISC License

All of these third-party licenses are compatible with the overarching MPL 2.0 framework. Any user can view the complete, exact list of licenses and the specific components they apply to by typing about:license into the Firefox address bar.

Source Code vs. Official Branding

While the underlying source code of Firefox is strictly open-source under the MPL 2.0 and compatible licenses, the official Firefox trademarks and logos are not distributed under an open-source license.

Mozilla retains exclusive rights to the Firefox name and artwork under the Mozilla Trademark Policy. This means that while anyone can download the Firefox source code, modify it, compile it, and distribute the resulting browser, they cannot call that compiled browser "Firefox" or use the official fox logo without explicit permission from Mozilla. To distribute a modified version of the code, developers must use the unbranded build configuration provided in the source code.