What License Does Firefox Use?
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.
Is Firefox free for commercial use?
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
Is Firefox proprietary or open source?
Mozilla Firefox or simply Firefox is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. Firefox uses the Gecko rendering engine to display web pages, which implements current and anticipated web standards.
Firefox - Wikipedia
Is Mozilla Public License 2.0 copyleft?
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