In part, it is because of a Rust component. A new CSS engine, Stylo, is shipping with 57 and that brought a noticeable performance improvement. In the foreseeab

In part, it is because of a Rust component. A new CSS engine, Stylo, is shipping with 57 and that brought a noticeable performance improvement. In the foreseeable future there will also be WebRender, which should give an even bigger improvement, but that wasn't ready for 57. Another big part is that they recently switched Firefox over to a multiprocess architecture (Firefox 48) and these performance improvements are now mostly optimizations that are now possible due to this architecture. Basically offloading all kinds of slower operations into a background thread. The optimizations are all over the place, mostly of smaller nature, but taken together they make a big difference. And a third point is the new UI. It is partially really just more efficient, partially uses animations and such to trick your brain into thinking that things are faster, and then it also now has "tab warm-up", which basically means that when you hover with your mouse over a tab, it already starts loading in that tab, so that it's instantly there when you click.