Mozilla has been getting everyone excited for Firefox 57, nicknamed 'Firefox Quantum,' which will bring a new interface and speed improvements to both the desktop and Android browsers. Firefox 57 isn't quite ready yet (you try it in beta, if you want), but version 56 has just been released with a few notable changes of its own.
First off, this release drops support for both Android 4.0 (4.1 or higher is now required) and the Adobe Flash plugin. Considering only about 0.6% of active devices run Android Ice Cream Sandwich, this won't have too much of an impact. ICS was also the last Android version that was (officially) supported by the Adobe Flash plugin, so now that Firefox doesn't run on Android 4.0, there's no reason to keep Flash around. Mozilla is on track to remove plugin support from desktop Firefox by 2020.
Firefox 56 also includes numerous improvements to WebExtensions on Android. If you're unaware, this is the new standard for Firefox add-ons, heavily based on Chrome's extension API. Several new APIs have been added, and some previously desktop-only functions have been ported to Android. There are still quite a few APIs not supported on Firefox for Android, but I'm sure Mozilla will continue to bridge the gap.
This update also includes a few new web features, hardware encoders for WebRTC on Android, and support for Zapoteco (zam) and Belarusian (be) locales. It's still rolling out on the Google Play Store, but you can download it from APKMirror right now.