Firefox's source code is open source, tri-licensed under the Mozilla Public License (MPL), GNU General Public License (GPL), and the GNU Lesser General Public License (LGPL). These licenses permit anyone to view, modify and/or redistribute the source code, and several publicly-released applications have been built on it; for example, Netscape, Flock and Songbird make use of code from Firefox.
The official end-user builds of Firefox distributed from mozilla.com are licensed under the Mozilla EULA. Several elements do not fall under the scope of the tri-license and have their use restricted by the EULA, including the trademarked Firefox name and artwork, and the proprietary Talkback crash reporter. Because of this and the clickwrap agreement included in the Windows version, the Free Software Foundation (FSF) consider these builds proprietary software.
In the past, Firefox was licensed solely under the MPL, which the FSF criticizes for being weak copyleft; the license permits, in limited ways, proprietary derivative works. Additionally, code under the MPL cannot legally be linked with code under the GPL or the LGPL. To address these concerns, Mozilla re-licensed Firefox under the tri-license scheme of MPL, GPL, and LGPL. Since the re-licensing, developers have been free to choose the license under which they will receive 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 choose the MPL.