The primary fix in v66.0.5 addressed a remaining issue where web extensions were still disabled for users with a master password set.
As part of the broader Firefox 66 release, this version includes: Mozilla Firefox Quantum v66.0.5
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. The primary fix in v66
A new ability to search through all open tabs via the tab overflow menu. released on May 7