Mozilla Public License 2.0 (MPL 2.0)

Overview

The Mozilla Public License (MPL) is an open source software license developed by the Mozilla Foundation that balances the characteristics of copyleft and permissive licenses.

Version: 2.0, January 3, 2012
Official URL: https://www.mozilla.org/en-US/MPL/2.0/

Core Features

MPL 2.0 is a file-level copyleft license, which means:

Modifications to MPL-licensed files must continue to use the MPL license
Can be combined with code under other licenses
More permissive than GPL, stricter than MIT/BSD

What You Can Do

✅ Use & Distribution

Commercial Use - Can be used in commercial products
Distribution - Can distribute in source or binary form
Modification - Can modify MPL-licensed code
Combination - Can combine with code under other licenses

✅ Patent Protection

Patent License - Automatic patent license from contributors
Patent Retaliation - Your license terminates if you sue anyone for patent infringement

✅ Relicensing

Under certain conditions, MPL code can be relicensed as:

GPL 2.0+ - If file header includes compatibility statement
LGPL 2.1+ - If file header includes compatibility statement
AGPL 3.0+ - If file header includes compatibility statement

Obligations and Requirements

📝 Source Code Disclosure

Modified MPL Files - Must release source code under MPL 2.0
Only Modified Files - Only need to release source for modified MPL files
Other Files Unaffected - Proprietary code can remain proprietary

📋 Information to Preserve

License Headers - Each MPL file must retain license header
Copyright Notices - Preserve all copyright and license notices
Change Records - Document significant modifications to files

🔄 Distribution Requirements

When distributing software containing MPL code:

Source Availability - Ensure source code of MPL files is available
License Notice - Include MPL license text
Change Description - Describe modifications to MPL files

Compatibility with Other Licenses

✅ Compatible Licenses

Can be combined with:

MIT/BSD - Permissive licenses
Apache 2.0 - Similar patent clauses
Proprietary Licenses - In separate files

🔄 Conditionally Compatible

GPL 2.0+ - Requires compatibility statement
LGPL 2.1+ - Requires compatibility statement
AGPL 3.0+ - Requires compatibility statement

❌ Incompatible Licenses

GPL 2.0 (only) - Without "or later" clause
Some Proprietary Licenses - May have conflicting terms

File-Level Copyleft Implications

📁 File-Level Rules

Individual Files - Each file considered independently for licensing
Modified Files - Modified MPL files must remain MPL-licensed
New Files - Newly added files can use different licenses

🔗 Linking and Combination

Static Linking - Allowed with proprietary code
Dynamic Linking - Allowed with proprietary code
Same Executable - MPL and proprietary code can be in same program

Patent Clauses

⚖️ Patent License

Automatic Grant - Contributors automatically grant patent license
Scope - Limited to patents necessary for their contributions
Defensive - Protects users from patent litigation

🛡️ Patent Retaliation

If you initiate patent litigation against anyone claiming MPL-licensed software infringes patents:

Your MPL license immediately terminates
Applies to all MPL 2.0 licensed software, not just the sued software

Practical Use Cases

✅ Projects Suitable for MPL 2.0

Libraries and Frameworks - Can be used by proprietary software
Development Tools - Compilers, editors, etc.
Enterprise Software Components - Need business-friendly copyleft
Mixed-License Projects - Partially open source, partially proprietary

⚠️ Scenarios to Consider

Pure Open Source Projects - GPL might be more appropriate
Simple Tools - MIT/BSD might be simpler
Web Services - AGPL might better prevent service circumvention

Compliance Guidelines

📦 When Distributing Binaries

Include License Text - Complete MPL 2.0 text
List MPL Files - Indicate which files use MPL license
Provide Source Code - Ensure source code of MPL files is available
Preserve Notices - Keep all copyright and license notices

💻 When Distributing Source Code

Retain File Headers - License header in each MPL file
Include License - Complete MPL 2.0 text
Mark Modifications - Clearly indicate modifications to files

🔧 When Modifying MPL Files

Maintain MPL License - Modified files must remain MPL 2.0
Add Copyright - Add copyright notice for significant modifications
Record Changes - Document modifications in file or changelog

Relationship with Other Mozilla Projects

🦊 Firefox

Firefox uses MPL 2.0, allowing:

Commercial distributions (like enterprise versions)
Proprietary extensions and plugins
Embedding in proprietary products

📧 Thunderbird

Similarly, Thunderbird's MPL licensing allows commercial use and customization.

Frequently Asked Questions

Q: Can I use MPL libraries in proprietary software? A: Yes, but you must maintain the copyleft nature of MPL files and provide source code for those files.

Q: If I modify an MPL file, must the entire project be open source? A: No, only the modified MPL files need to be open source; other files can remain proprietary.

Q: What's the main difference between MPL and GPL? A: MPL is file-level copyleft, GPL is project-level copyleft. MPL allows easier combination with proprietary code.

Q: Do I need to pay licensing fees for using MPL libraries? A: No, MPL is a free open source license.

Best Practices

📋 License Management

Maintain Inventory - Track all MPL files
Automated Checking - Use tools to verify license compliance
Train Team - Ensure developers understand MPL requirements

🔍 Code Review

Check New Files - Ensure proper license headers
Verify Modifications - Ensure modified MPL files remain compliant
Document Decisions - Record reasons for license choices

Legal Notice

This document is a simplified explanation of MPL 2.0 and has no legal force. Please read the complete license text carefully before use.

The Mozilla Foundation is not a law firm and does not provide legal services. Distributing, displaying, or linking to this summary does not create a lawyer-client relationship.