It's not an opinion that you'll lose potential customers if your site doesn't work in THEIR browser of choice. That's fact.
That is a ridiculous assertion. One example from the real world involves online college courses. If the college uses something that doesn't support FF what are the students going to do? Go to another school? NO, they'll use IE as indicated the requirements. You use the textbooks specified. You do the assignments. You use the browser you are told to. Going to another school because you get all pissy about having to use IE doesn't sound like an adult way to handle things. I have other examples if you'd like.