A new ability to search through all open tabs via the tab overflow menu.
As part of the broader Firefox 66 release, this version includes:
Allows users to authenticate on websites using facial recognition or fingerprints on Windows 10. System Requirements Firefox 66.0.5 System Requirements
Prevents web pages from "jumping" while images and advertisements load.
Mozilla Firefox Quantum v66.0.5, released on May 7, 2019, was a critical point update primarily focused on resolving a significant bug that caused browser extensions to be disabled.
Automatically prevents websites from playing sound-enabled media.