The rendering engine used by Firefox is Gecko.
Gecko is an open-source browser engine developed by Netscape and later maintained primarily by Mozilla. It handles the work required to turn web technologies such as HTML, CSS, SVG, and MathML into the pages users see and interact with. Firefox uses Gecko for both web-page rendering and parts of its browser interface.
Gecko originated in the late 1990s as Netscape’s next-generation layout technology. After Netscape released much of its browser code as open source, the Mozilla project continued development. Gecko became the foundation for Mozilla products, including Firefox, Thunderbird, and SeaMonkey.
A frequent confusion is mixing up a browser engine with a JavaScript engine. Gecko is responsible for layout and rendering, while Firefox’s JavaScript execution is handled by SpiderMonkey. Likewise, Trident was associated with Internet Explorer, WebKit powers or powered browsers such as Safari and older Chrome-based products, and Blink is the engine used by Chromium-based browsers. Modern Gecko also incorporates technology from Mozilla’s Servo project.