Licensing
Firefox source code is free software, with most of it being released under the Mozilla Public License (MPL). This license permits anyone to view, modify, and/or redistribute the source code, and several publicly released applications have been built on it; for example, Netscape, Flock, Miro, Iceweasel, and Songbird make use of code from Firefox.
In the past, Firefox was licensed solely under the MPL, which the FSF (Free Software Foundation) criticized for being weak copyleft; the license permitted, in limited ways, proprietary derivative works. Additionally, code only licensed under the MPL could not legally be linked with code under the GPL. To address these concerns, Mozilla re-licensed most of Firefox under the tri-license scheme of MPL, GPL, or LGPL. Since the re-licensing, developers were free to choose the license under which they received most of the code, to suit their intended use: GPL or LGPL linking and derivative works when one of those licenses is chosen, or MPL use (including the possibility of proprietary derivative works) if they chose the MPL. However, on January 3, 2012, Mozilla released the GPL-compatible MPL 2.0, and with the release of Firefox 13 on June 5, 2012, Mozilla used it to replace the tri-licensing scheme.
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