undefined. For example,
window.x.y // Firefox 62: //// TypeError: window.x is undefined // Firefox 63: //// TypeError: window.x is undefined, can't access property "y" of it // Firefox 64: //// TypeError: window.x is undefined; can't access its "y" property
Given that the format of error messages is not standardized in the ECMAScript spec, web developers are discouraged to use them programmatically.
Update: Flipkart is known to be affected by this change.
Updated 2 This change has been backed out from Firefox 63 Release Candidate 2.