In the new in-content preferences, unchecking a checkbox confuses me. What I would expect is that I'd click the box, and the check would disappear. What actually happens is that the whole box, including the checkmark, dims, appearing to disable itself. This is due to the mouseover state, which looks like a box with a dimmed checkmark. (At least on the Mac, a dimmed, checked checkbox is the visual vocabulary for "This option is on, but you cannot turn it off.")

This was fixed by bug 996036.