ever since they've switched to the quantum engine it also feels really fast, almost faster than chrome with the exception of youtube, which feels sluggish sadly.
That's because YouTube uses non standard HTML features that only work in chrome and then polyfills it for every other browser so it works like shit unless you use chrome.
If YouTube page loads are slow for you in Firefox, you can install the "YouTube Classic" extension to opt-out of YouTube's (2017) Polymer design. This extension doesn't affect video playback, just the page layout.