This is something I’ve noticed as well, not specifically related to Cesium, but just with WebGL applications in general. There might be some specific things we can do to improve performance, and this is something we’ve been looking more into recently, but these implementation differences across browsers are generally outside of the application developer’s control.
One thing that might help is running some benchmarks, perhaps with the Firefox/Chrome profilers respectively, to figure out what exactly is causing the most slowdown, and if it’s something on the CesiumJS side or on the browser side (or in your code).