Less common web browsers are going to score higher simply because only nerds know they even exist :p. As more and more normal people use a browser, the more towards the average worldwide IQ it would fall. I bet Firefox had a much higher score when it was in version less-than-1 :p.

I do find it strange that IE9 scores lower than IE8, though. What could the reason be?

IE8 users aren't scoring terribly bad anyway, considering the average IQ in the US is 98, they are just like 2-3 points below, and you'd have a hard time telling a 96 iq person from someone with 98 just by talking to them.