What License Is Firefox Under?
Mozilla Public License - Mozilla is the custodian of the Mozilla Public License ("MPL"), an open source/free software license. The current version of the license is MPL 2.0 (html | plain text). If you want to use or distribute code licensed under the MPL 2.0 and have questions about it, you may want to read the FAQ.
It is a weak copyleft license, characterized as a middle ground between permissive software licenses and the GNU General Public License (GPL), that seeks to balance the concerns of proprietary and open-source developers. ...
Mozilla Public License - Wikipedia
Mozilla Firefox is free and open source software, built by a community of thousands from all over the world. There are a few things you should know: Firefox is made available to you under the terms of the Mozilla Public License. This means you may use, copy and distribute Firefox to others.
Firefox: About Your Rights - Mozilla
The MPL is a simple copyleft license. The MPL's "file-level" copyleft is designed to encourage contributors to share modifications they make to your code, while still allowing them to combine your code with code under other licenses (open or proprietary) with minimal restrictions.
MPL 2.0 FAQ - Mozilla