Firefox 57 is easily the best update to come to Firefox ever. Mozilla even named this release “Quantum” because of how dramatically the browser speed improved. Couple that with the complete redesign of Firefox’s UI, and you have a pretty dramatic change.
Thanks to Debian’s release schedule, it’s unlikely that Stretch will ever officially see Firefox 57. However, it is available in Sid. You won’t need to upgrade all the way to Sid to get it either. You only need to tell Debian to use Firefox from Sid.