The first is correct. The second should be the same. This is true for all the built-in *Error prototypes that inherit from Error.prototype.
Classifying as trivial because it's hard to imagine anything would care. If closed as wont-fix, please tag as DeliberateSpecViolation or some such so we can track.
Created attachment 101048 [details]
Fixed in r91116
*** Bug 41602 has been marked as a duplicate of this bug. ***