Gecko (software)

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Gecko is Mozilla’s browser engine. It powers Firefox and Thunderbird and has been used in other Mozilla projects and various Netscape-era products. Gecko renders web pages and can also render an app’s user interface using XUL. It’s designed to support open web standards and is available as free, open-source software under the Mozilla Public License 2.0. Gecko runs on Android, Linux, macOS, and Windows. It’s written mainly in C++, with JavaScript and, since 2016, Rust.

Gecko’s history began in 1997 at Netscape to replace an older engine that didn’t fully meet standards. The code was open-sourced in 1998 as NGLayout, later renamed Gecko. Netscape publicly announced it would switch to Gecko in 1998–2000, and Netscape 6 (the first to use Gecko) appeared in 2000. AOL laid off many Gecko developers in 2003, and the Mozilla Foundation became the main steward of development. Over the years Gecko work faced performance and bloat challenges, which helped prompt Apple to fork WebKit for Safari in 2003, while Mozilla continued improving Gecko. By 2008 Gecko’s performance had notably improved.

In 2016 Mozilla launched Quantum, a project to build a faster, next‑generation web engine by incorporating ideas from the Servo project (a separate experiment written in Rust). Firefox 57, released in 2017, was the first major version to include many Quantum/Servo components, boosting CSS and GPU rendering. In 2018 Mozilla introduced GeckoView to make Gecko reusable for Android apps, and it powered Firefox Focus and Firefox Reality. Firefox Preview followed in 2019 to explore an Android browser built around GeckoView, and Firefox for Android Daylight (2020) used it as its core.

Gecko supports features such as DOCTYPE switching, which lets pages render in standards mode or quirks mode depending on the document type. It’s primarily used in Firefox and Thunderbird, but several forks and derivatives also rely on Gecko, including LibreWolf, IceCat, Waterfox, Pale Moon (via Goanna), Lunascape, and Sailfish Browser. KaiOS also uses a Gecko-based stack. Some older projects have switched to other engines over time.

Gecko’s versioning synced with Firefox and Thunderbird; after Gecko 2.0, the line was bumped to 5.0 to align with Firefox 5, and the major Gecko version generally tracks the corresponding major Firefox/Thunderbird release.

In 2012 Mozilla started Servo, a Rust-based engine focused on safety and parallelism. Quantum, announced in 2016, brought stable parts of Servo into Gecko to improve performance, especially for CSS and graphics. The Servo team was reduced in 2020, but Quantum and related efforts continue to shape Gecko. Another related effort, Azure, provides a modern graphics API for Firefox.

Gecko’s stable release as of November 4, 2022 was version 125.