> That's true; I had an impression it's done on purpose to support Mozilla non-standard const

Same here, but it's all wrong and will require a non-trivial amount of refactoring.

> I'd be happy to make const work more or less let.

I would love that, but for something this drastic @valueof needs to be involved.

> Still, both of them interpret the Temporal Dead Zone incorrectly and if valid code that didn't break starts breaking, it's a regression.

Yes, it needs to be addressed ASAP to limit the amount of code that could potentially become invalidated.