I've seen Firefox do this on occasion, but never seen a router do it. Occasionally instead of displaying a page, Firefox will display the code and other times will throw up a login window somewhat like this. But I've loaded all our forums and pages countless times through a Linksys router and never seen this issue.
Yes I saw that which is why this makes no sense. That is definitely a Linksys model number in that login window. Since it's a very popular brand, perhaps the hijacking Mal ware is just playing the percentages