Compatibility With Other Licenses
Unlike strong copyleft licenses, code under the MPL may be combined with proprietary files in a "larger work" so long as conditions for the MPL are still met for "covered" components (Section 3.3 of the license). The MPL treats the source code file as the boundary between MPL-licensed and proprietary parts, meaning that all or none of the code in a given source file falls under the MPL.
MPL version 2.0 is compatible with both the Apache License and GPL, but version 1.1 had "some complex restrictions" that made it incompatible with the GPL by default. Although the MPL 1.1 did include a provision (Section 13) for providing a work under a secondary license (including the GPL or GPL-compatible ones), MPL 1.1 and GPL code could not "legally be linked," leading the Free Software Foundation to discourage using the MPL 1.1. For these reasons, earlier versions of the Mozilla Suite and Firefox were released under multiple licenses: the MPL, GPL, and LGPL.
Read more about this topic: Mozilla Public License