With this release, "legacy" add-ons (XUL-based) will no longer function. This update changes the add-ons system to the WebExtensions API. The Mozilla Add-ons portal will list only WebExtensions-compatible add-ons by default. Legacy Extensions are listed separately located under Tools > Add-ons. From there click "Find a Replacement"and check the three pages of available extensions.
In addition, this update introduces the new Quantum engine (Firefox Quantum) which is replacing parts of parts of the familiar old Gecko engine.
Mozilla Firefox Release Notes