I took it as him just copying something he found somewhere online without knowing that it would only work in MSIE. I know very few examples when something was developed for MSIE and MSIE only – all of which were professional enterprise solutions*.
I think it's not safe to infer from one line of code of a beginner that he wanted to go for a MSIE only solution. Even more so because he was using FireFox. Why would he wonder that it doesn't work in FF if he knew it's MSIE only?
*) Although you could argue that MSIE and "professional" are opposites.