If the kid didn’t install Firefox (not impossible, but not likely in a school; if he could install a program on a school computer, the school has a serious IT problem), and if he was told to use the Internet to do research, he didn’t do enough to warrant suspension.
The assignment wasn’t to use Internet Explorer; it was to do research on the Internet.
And the kid was told THREE TIMES not to use Firefox before he was given detention. All he needed to do was close Firefox and use IE. The ONLY issue that matters is that the kid was disobedient, the teacher's lack of tech savvy has nothing to do with it.
I wonder what the reaction would be if it was reversed. What if the kid was using IE, but was supposed to be using Firefox. My guess is that the story wouldn't have gotten a bit of notice.