When you’re a sale person, and you’re making a sales call, never insult the potential customer by saying “Well, we didn’t test with foo because you’re the only person in the world who uses it!”
In this case foo was Firefox after a vendor’s webapp’s magic javascript didn’t work with it. Yes, IE has the marketshare and is the most important to get working. However, have a better response then “you’re a freaking weirdo,” which is about what he said.