I just tried Firefox 57 on my Windows machine. One of the first pages it showed me was https://www.mozilla.org/en-US/privacy/firefox. Scrolling on that page I immediately noticed that it was dropping frames, a lot. I tried the same page in Chrome and it maintains 60 FPS. I've got high spec machine but with an NVidia NVS 310, which might be the problem. However, Chrome is fine...