GLOSSARY . DEFINITION
What Is the Mozilla Public License?
The Mozilla Public License, often shortened to MPL, is an approved open source license with a file level copyleft. If you change a file it covers, you share the source of that file under the same license, but you may combine those files with proprietary code in a larger work without licensing the whole work. For enterprises, the Mozilla Public License sits in the middle ground between permissive licenses and the strong copyleft of the GPL.
Definition
The Mozilla Public License is an open source license maintained by the Mozilla Foundation, currently at version 2.0, and approved by the Open Source Initiative. Its defining feature is file level copyleft, sometimes called weak copyleft. The obligation attaches to individual source files rather than to the whole program. If you modify a file that is licensed under the Mozilla Public License, you must make the source of that modified file available under the same license. Files you add that contain your own proprietary code are not covered, even when compiled together into one product, provided the covered and proprietary code stay in separate files.
How it compares to other licenses
The Mozilla Public License sits between two poles. Permissive licenses such as Apache 2.0 and the MIT License impose almost no sharing obligation, letting you take the code and keep your changes private. The GPL family applies strong copyleft to the entire combined work, which can require releasing your whole program under the GPL if you link GPL code into it. The Mozilla Public License draws the line at the file. Your changes to covered files are shared, your separate proprietary files are not. This makes it materially easier to combine with proprietary software than the GPL, while still ensuring improvements to the covered files flow back. The broader distinctions are set out in permissive versus copyleft versus source available explained.
Why it matters to enterprises
The Mozilla Public License is a stable, approved open source license, not a source available license like the Business Source License or the Server Side Public License, so it carries no competitive use restriction. Its risk is a compliance one: if your teams modify covered files, the obligation to share those modifications is real, and it is easy to lose track of in a large codebase. The Mozilla Public License also appears as the conversion target for some Business Source License releases, which means a component you watch for relicensing reasons may settle into the Mozilla Public License after its delay. Knowing where it sits in your dependency tree, and which covered files your teams have touched, is ordinary license risk hygiene. It is the kind of detail a dependency map surfaces and an assumption misses. More terms are defined in the open source license risk glossary, and the wider reason source available licenses differ from this one is covered in why source available is not open source.
This definition is commercial and licensing risk context, not legal advice. For interpretation of the Mozilla Public License against your specific use, engage your own counsel.
COMMON QUESTIONS
Questions buyers ask.
What is the Mozilla Public License?
The Mozilla Public License, or MPL, is an Open Source Initiative approved open source license with a file level copyleft. If you modify a file covered by the Mozilla Public License, you must make the source of that file available under the same license, but you may combine those files with proprietary code in a larger work without licensing the whole work. The current version is MPL 2.0.
Is the Mozilla Public License copyleft?
Yes, but at the file level rather than the whole work. The Mozilla Public License is often called a weak copyleft license. Changes to covered files must be shared under the same license, while proprietary code in separate files can remain proprietary. This sits between permissive licenses and the strong copyleft of the GPL family.
How does the Mozilla Public License differ from the GPL?
The GPL applies copyleft to the entire combined work, so linking GPL code into your software can require releasing the whole work under the GPL. The Mozilla Public License applies copyleft only to the covered files, so proprietary code kept in separate files is not pulled in. The Mozilla Public License is generally easier to combine with proprietary software.
Is the Mozilla Public License a relicensing risk?
The Mozilla Public License itself is a stable, approved open source license and is not a source available license like the Business Source License or the Server Side Public License. It carries an obligation to share modifications to covered files, which is a compliance point rather than a competitive use restriction. Knowing where it sits in your dependency tree is part of ordinary license risk hygiene.
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