Oh no, Ingolme!!!
It seems to be an interesting behaviour of window.getComputedStyle() and getPropertyValue("text-decoration")
Its important to note, that the exercise checks for the existence of none inside this property value.
On Firefox, when you've defined the text-decoration as none it actually removes the text-decoration-line(default: underline) and text-decoration-style(solid) keywords from the text-decoration property value. So you're only left with the text-decoration-color. Hence the process fails.
On Chrome, text-decoration is property that seems to be directly combined from text-decoration-line, text-decoration-style, and text-decoration-color. Hence none exists, the process succeeds, and the question is marked correct.
If the question checked for text-decoration-line instead of generic text-decoration, this question would no longer be an issue.
I agree, this is very interesting stuff!