The Mozilla Public License (MPL) is an open-source software license best known for its “file-level” or “weak” copyleft. It occupies a deliberate middle ground between permissive licenses such as the BSD and MIT licenses, which impose almost no obligations on redistributors, and strong copyleft licenses such as the GNU General Public License, which require that an entire derived work be released under the same terms. The MPL requires only that the specific files containing MPL-covered code remain under the MPL, while permitting those files to be combined with code under other licenses, including proprietary code.
The license originated from Netscape’s 1998 decision to release the source code of its Communicator browser, the event that launched the Mozilla project. Netscape initially published two licenses, the Netscape Public License and the original Mozilla Public License, to govern the newly opened code. The MPL was crafted to let a commercial codebase be opened to community development while still allowing the originating company and others to build proprietary products on top of it, a balance that strong copyleft would not have permitted.
The current version, MPL 2.0, was released by the Mozilla Foundation in January 2012 as a modernization and simplification of the earlier text. Its core mechanism appears in the requirement that “all distribution of Covered Software in Source Code Form, including any Modifications that You create or to which You contribute, must be under the terms of this License.” Because the unit of copyleft is the individual source file rather than the whole program, a developer can add MPL-covered files to a larger work without that larger work becoming subject to the MPL.
MPL 2.0 also includes an explicit patent license from each contributor, drawn from the patent claims they would necessarily infringe by their own contributions, and provides defensive termination of those patent grants if a licensee initiates patent litigation over the software. The 2.0 revision improved compatibility with the GPL and the Apache License, addressing a longstanding friction point where MPL-licensed and GPL-licensed code could not easily be combined. Software distributed in executable form must still be accompanied by, or offer access to, the corresponding source code.
The MPL is used by Firefox, Thunderbird, and a wide range of other projects, and it serves as a frequently cited template for organizations that want genuine copyleft protections on their own files without forcing copyleft onto everything those files touch. As a primary artifact of the open-source movement, the license text itself, published and maintained by Mozilla, is the authoritative reference for its terms, and the move from version 1.1 to 2.0 is a notable example of a major project iterating on its own legal foundations to improve interoperability across the licensing ecosystem.