The site listed (source here: https://github.com/firefox-is-a-good-browser/firefox-is-a-good-browser.github.io) uses a large amount of memory and CPU, which persists when the tab is closed. The relevant part seems to be the following CSS: a { display: block; position: absolute; left: 50%; top: 50%; transform: translate(-50%, -50%); animation: a 1s ease-in-out 0s infinite alternate; opacity: .99; background: #fff url("data:image/jpeg;base64,--ommitted--") center/100% 100% } where the jpeg is actually a 10000x10000 png (that displays fine in firefox). My wild guess is that the animation is creating frames faster than layers can consume them, but I haven't looked into it.

This looks to be a very bad case of bug 1243446/bug 1139928.