I have an exact hat like that, except that it's not orange in colour. In fact, I shall wear it right now.
You want to know how old it is? 12 years old. I know because it was the first piece of Winter clothing that I never got handed down to me. I bought it myself.
And even worse, it's a Canadian artefact, it's a Tuque! (Pretty sure it's worn around the world, but in Canada, it's pretty a common design.) We've been making these for at least a hundred years.
Firefly had two things going for it's popularity, it was riding off the coat tails of Joss Whedon's other properties (And after Angel and Buffy, his name had mass appeal) and Fox's screwing with it, giving it a mystique and allure that it might not have gotten otherwise.
And Serenity had enough silly things in it that broke any sense of immersion I could have gotten out of it (and this is coming from a Space Opera fan) and I still don't get it's popularity.