MPL

Mozilla published it on 3 January 2012 after a two-year public revision of version 1.1. The Mozilla Foundation is the license steward. SPDX lists the short identifier MPL-2.0 for the standard form: the Exhibit A source-code notice, without Exhibit B.

It is a file-level copyleft license for Free and Open Source Software (FOSS). Modified files must stay under these terms. Files you add that contain no Covered Software may be proprietary, even when they ship in the same product.

The MPL fills a useful space in the spectrum of free and open source software licenses, sitting between the Apache license, which does not require modifications to be shared, and the GNU family of licenses, which requires modifications to be shared under a much broader set of circumstances than the MPL.

Mozilla, MPL 2.0 FAQ

Mozilla’s source-code policy keeps modifications to Product Code (files that ship in nightly binaries of client-side Mozilla products) under these terms, and recommends this license for new client-side projects. New Mozilla Code may instead use Apache-2.0. Mozilla-provided Firefox executables already satisfy Section 3’s source-availability and notice rules.

File-Level Copyleft

Copyleft stops at the file. Recipients of Covered Software keep the right to use, modify, and redistribute those files. They do not have to open the rest of a proprietary product built around them. That is the difference from whole-work copyleft under the General Public License (GPL), and from permissive OSS licenses that never require modifications to be shared.

• Covered Software: Source Code Form that carries the Exhibit A notice, the Executable Form of that source, and Modifications, including any new source file that contains Covered Software (Sections 1.4 and 1.10).
• Source Code Form: Section 3.1 requires every distribution of Covered Software in source form, including your Modifications, to stay under this license. Recipients must be told that the source is governed by these terms and how to obtain a copy of the license. You may not restrict their rights in the source.
• Executable Form: Section 3.2 requires you to make the corresponding Source Code Form available by reasonable means, in a timely manner, at no more than the cost of distribution. You may sublicense the executable under different terms, including a commercial End-User License Agreement (EULA), provided those terms do not limit or alter source rights. Minified JavaScript is Executable Form, not Source Code Form.
• Larger Work: Section 1.7 defines a Larger Work as Covered Software combined with other material, in separate files, that is not Covered Software. Section 3.3 lets you distribute that combination under terms of your choice for the non-covered files. Proprietary files may be statically linked so long as the MPL files remain available under these terms.
• Distribution versus network use: Private modification and distribution inside an organization do not trigger the outbound duties. Software as a Service (SaaS) that never delivers a copy of server-side code is not distribution. HTML, CSS, and JavaScript sent to the browser are.

Patent Grant and Secondary Licenses

Version 2.0 added an express patent grant and a standardized path into the GNU family, so projects no longer need ad hoc dual- or tri-licenses for the usual combination cases.

• Express patent grant: Section 2.1(b) grants a worldwide, royalty-free, non-exclusive license under each Contributor’s Patent Claims to make, use, sell, offer for sale, have made, import, and otherwise transfer its Contributions or Contributor Version.
• Grant limits: Section 2.3 withholds a patent license for code a Contributor removed, for infringements caused by third-party modifications or combinations outside the Contributor Version, and for claims infringed by Covered Software in the absence of that Contributor’s Contributions. There is no trademark license except as needed to keep required notices intact.
• Litigation termination: Section 5.2 terminates the Section 2.1 grants from every Contributor if you initiate litigation alleging that a Contributor Version infringes a patent (excluding declaratory judgments, counter-claims, and cross-claims).
• Secondary licenses: Section 1.12 defines Secondary License as GPL, LGPL, AGPL, or any later versions of those licenses (including GPL and LGPL). Section 3.3 lets you additionally distribute Covered Software under those terms when you combine it with a work already under a Secondary License, unless the code is Incompatible With Secondary Licenses.
• Exhibit B opt-out: Attach Exhibit B to mark Covered Software Incompatible With Secondary Licenses. That is an active choice. The default SPDX identifier MPL-2.0 is Exhibit A without Exhibit B. Code that was version 1.1 only, and not also under a Secondary License, is incompatible when upgraded to 2.0 (Section 1.5(b)).

The revision also made compliance with this license satisfy Apache-2.0’s terms, so Apache-licensed files can sit in the same binary as MPL files.

Compared With GPL, LGPL, Apache, EPL, and CDDL

Choosealicense classifies this as a weak copyleft license: commercial use, modification, distribution, private use, and patent use are granted, conditioned on disclosing source for licensed files, keeping copyright and license notices, and applying the same license at file scope. Liability, trademark use, and warranty are disclaimed. The table places that file-level bargain against neighboring licenses.

License	Copyleft scope	Proprietary combination	Patent grant	GNU-family compatibility
MPL-2.0	File-level (weak). Modified MPL files stay MPL.	Yes. Other files in a Larger Work may be proprietary.	Yes (Section 2.1(b)). Terminates if you sue over a Contributor Version.	Default yes. Exhibit B opts out.
GPL	Whole-work (strong).	No. The derivative as a whole must be GPL.	Explicit in GPL.	The GNU copyleft family.
LGPL / LGPL	Library-level (weak).	Linking a proprietary application is allowed if relinking remains possible.	Explicit in LGPL-3.0.	Designed to sit under the GPL.
Apache-2.0	None (permissive).	Yes. Modifications need not be shared.	Yes.	Compatible with GPL-3.0, not with GPL-2.0.
EPL	Weak (Modified Works). Binaries may use other terms.	Yes.	Yes.	Secondary License is opt-in via Exhibit A (GPL-2.0 or later).
CDDL	File-level (weak), without MPL 2.0’s secondary-license clause.	Yes. Other files in a Larger Work may use other terms.	Yes.	No GNU secondary-license bridge comparable to MPL 2.0.

Where the GPL copylefts the whole work and Apache-2.0 copylefts nothing, this license copylefts the file. LGPL and LGPL instead copyleft the library while allowing proprietary linking. CDDL is also file-level copyleft, without this license’s GNU secondary-license clause. EPL is weak copyleft too, but its GNU compatibility is opt-in rather than opt-out.