Yes, I noticed it too. Had feedback from visually impaired users that the distiction between checked and not wasn't sufficient.
Most other browsers default styling (and indeed, earlier versions of Firefox) is far more obvious if it's checked or not, and I'd rather not have to explicitly style things in CSS, and give the users the experience they're used to seeing in their web browser on their platform.. but for the visually impaired, the very slim difference between the current 'checked' and 'unchecked' states needs to be far more obvious.
