Gecko is a web browser engine used in many applications developed by Mozilla Foundation and the Mozilla Corporation (notably the Firefox web browser including its mobile version other than iOS devices, and their e-mail client Thunderbird), as well as in many other open source software projects. Gecko is free and open-source software subject to the terms of the Mozilla Public License version 2.
It is designed to support open Internet standards, and is used by different applications to display web pages and, in some cases, an application’s user interface itself (by rendering XUL). Gecko offers a rich programming API that makes it suitable for a wide variety of roles in Internet-enabled applications, such as web browsers, content presentation, and client/server.
Gecko is written in C++ and is cross-platform, and runs on various operating systems including BSDs, Linux, macOS, Solaris, OS/2, AIX, Open VMS, and Microsoft Windows. Its development is now overseen by the Mozilla Foundation.
Gecko is primarily used in web browsers, the earliest being Netscape 6 and Mozilla Suite (later renamed SeaMonkey). It is also used in other Mozilla web browser derivatives such as Firefox and Firefox for mobile and the implementation of the Internet Explorer-clone that is part of Wine. Mozilla also uses it in theirThunderbird email-client and their Firefox OS.
Other web browsers using Gecko include Airfox, Waterfox, K-Meleon, Lunascape, Pale Moon, Portable Firefox, Conkeror, Classilla, TenFourFox, HP Secure Web Browser, Oxygen and Sylera (for mobile).
Other products using Gecko include Nightingale, Instantbird and Google’s picture-organization software Picasa (for Linux).
DevHelp, a GTK+/GNOME browser for API documentation, used Gecko for rendering documents.
Gecko is also used by Sugar for the OLPC XO-1 computer. Gecko is used as a complete implementation of the XUL (XML User Interface Language). Gecko currently defines the XUL specification.
Products that have historically used Gecko include Songbird, Epiphany (now known as Web and no longer using Gecko), Sunbird (calendar), and other web browsers including Swiftfox, Flock, Galeon, Camino, Minimo, Beonex Communicator, Kazehakase, and MicroB.
On Windows and similar platforms, Gecko depends on proprietary compilers. Thus, FOSS distributions of Linux can not include the Gecko package used in the Windows compatibility layer Wine.
After Gecko 2.0, the version number was bumped to 5.0 to match Firefox 5, and from then on has been kept in sync with the major version number for both Firefox and Thunderbird, to reflect the fact that it is no longer a separate component.
In October 2016 Mozilla announced the Quantum project, which is “the next-generation web engine for Firefox users, building on the Gecko engine as a solid foundation”. The plan includes replacing two large Gecko components with those of Mozilla’s experimental Servo engine. Mozilla hopes to complete this work by the end of 2017.