Have you ever been on Facebook or Twitter, merrily scrolling down the page, when all of a sudden, the browser appears to freeze? For several long seconds it just hangs there, and you’re not sure if it’s going to crash or not. Then, finally, something gives way, and the page jumps to catch up to your scrolling, disorienting you?
Yes, Facebook is particularly bad.
That’s called jank, and in Firefox 46 Beta we are well on our way to making jank a thing of the past.