[Sylos]

It mostly feels smoother, i.e. less laggy. To me, that's most noticeable when scrolling, which comes from the added implementation of Asynchronous Panning and Zooming (APZ).

Basically, beforehand when you turned your scroll wheel, Firefox first sent a scroll event to the webpage, then the webpage could execute whatever it needed to execute on scroll, and then Firefox actually scrolled the page. With APZ, that's now handled in parallel, i.e. it already scrolls what's there and alters it at the same time with whatever the webpage wants to do on scroll.

This also applies for zooming the page, as you might have guessed from the name, and as far as I can tell also to resizing the window.

Other than that, all animations in the browser UI are a lot smoother, too. For example, I can now just hold down Ctrl+T (which opens new tabs) and it happily chugs through playing the tab-animation hundreds of times with seemingly the only limiting factor being my screen's refresh rate. (And that's on a below-average laptop.)