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.

The existing Netscape rendering engine, originally written for Netscape Navigator 1.0 and upgraded through the years, was slow, did not comply well with W3C standards, had limited support for dynamic HTML and lacked features such as incremental reflow (when the layout engine rearranges elements on the screen as new data is downloaded and added to the page).

[citation needed] In October 1998, Netscape announced that its next browser would use Gecko (which was still called NGLayout at the time) rather than the old layout engine, requiring large parts of the application to be rewritten.

While this decision was popular with web standards advocates, it was largely unpopular with Netscape developers, who were unhappy with the six months given for the rewrite.

America Online, by this time Netscape's parent company, eventually adopted it for use in CompuServe 7.0 and AOL for Mac OS X (these products had previously embedded Internet Explorer).

[citation needed] In the Netscape era, a combination of poor technical and management decisions resulted in Gecko software bloat.

[14] In October 2016, Mozilla announced Quantum, an ongoing project encompassing several software development efforts to "build the next-generation web engine for Firefox users".

[15] In September 2018, Mozilla announced GeckoView, the foundation of Mozilla's next generation of mobile products based on a software library that makes Gecko reusable for Android, encompassing newer software development efforts to "decouple the engine itself from its user interface, and made it easy to embed in other applications".

Firefox Focus 7.0, shipped in the same month,[17] is the initial version introduced GeckoView, with increased performance in median page loading.

[18] In June 2019, Mozilla announced Firefox Preview as an ongoing project that focuses on building an Android web browser with GeckoView.

Other web browsers using Gecko include GNU IceCat,[26]Midori[27], Waterfox, K-Meleon, Lunascape, Portable Firefox, Conkeror, Classilla, TenFourFox.

Discontinued products that used Gecko include Swiftfox, Flock, Galeon, Camino, Minimo, Beonex Communicator, Kazehakase, Songbird, Sunbird (calendar), MicroB, Nightingale, Instantbird, and Picasa for Linux.

[33] Quantum is a Mozilla project encompassing several software development efforts to "build the next-generation web engine for Firefox users".

In 2012, Mozilla began the experimental Servo project, which is an engine designed from scratch with the goals of improving concurrency and parallelism while also reducing memory safety vulnerabilities.