Web browsers need software called a layout engine to interpret HTML , CSS , JavaScript , and embedded content (like images) and draw everything to your screen. Besides this, Gecko makes sure associated APIs work well on every operating system Gecko supports, and that appropriate APIs are exposed only to relevant support targets. This means that Gecko includes, among other things, a networking stack, graphics stack, layout engine, a JavaScript virtual machine, and porting layers.

Since all Firefox OS apps are Web apps, Firefox OS uses Gecko as its app runtime as well.