I've been using the new Firefox beta (v57) for about a month now. It's the first version I've liked more than Chrome - not only is the UI design way better than it used to be, but it's also super responsive and light on memory. Don't regret switching at all.
Yea, they dragged their feet for about 2 years or so, but this effort more than made up for it. They're not even done with the speed improvements, yet! There is another phase that wasn't ready in time for 57.
This is great! I like that they even have an easing function they suggest using to keep animations consistent. Although, I'd like to see some Sketch components or a base CSS file for new projects to start with.
Firefox's add-ons section looks stagnant. I hope this encourages developer to update/create new ones to look as modern as Chrome's.