Mozilla Firefox 57 has been released today, though the official server does not have the version at the moment. It will be available for download later today, but the latest version is available from various channels.
Firefox 57 marks the arrival of the new Quantum engine, which according to Mozilla is twice as fast as previous versions, and uses 30% lesser memory (RAM) than Google Chrome. We have been testing the final version for a day, and it seems pretty snappy. 
Firefox 57.0 also brings in a new interface design, called Photon UI, which we have reported about in the past. It replaces Australis (from v29.0), into something more pleasant.
WebExtensions are here, and this means the end of legacy add-ons. This is both good and bad, as in developers can build their add-ons for cross-browser support easily (Chrome, Edge, Firefox, etc), but many old add-ons cannot be re-written to worh with the new API.
Popular WebExtensions which are compatible with Firefox 57:
- Adblock Plus
- Checker Plus for Gmail
- Chrome Store Foxified
- Ghostery
- Greasemonkey
- HTTPS Everywhere
- IDM Integration Module (Internet Download Manager)
- Keepa.com - Amazon Price Tracker
- LastPass Password Manager
- Reddit Enhancement Suite
- Ublock Origin
- Xmarks Booomarks Sync
List of popular add-ons which are not compatible with Firefox 57 or above:
- All-in-One Sidebar (Will not be updated)
- DownThemAll!
- FEBE by Chuck Baker: (Will not be updated)
- gtranslate
- OmniSidebar (Will not be updated)
- Session Manager by Michael Kraft
- Speed Dial by Josep del Rio (Not updated since 2015)
- Tab-Mix Plus
List of popular add-ons which may be updated for WebExtensions: (an update is being planned or discussed for the following add-ons)
- DownThemAll! Lite
- gtranslate by Pau Tomàs, Pierre Bertet, Éric Lemoine
- NoScript
- Session Manager by Michael Kraft
- Tab-Mix Plus
Alternatives for Firefox add-ons which are not compatible with Quantum:
- Group Speed Dial: (can be used to import dials from Speed Dial by Josep del Rio)
- S3.Translator: Decent alternative for gtranslate
We will update this list when we come across more information.
Download Mozilla Firefox 57.0 stable version from the official FTP servers now, or wait for the update to be made available later at the Mozilla Firefox website today.