Well, that sucks. I have not used IE is ages and a healthy chunk of my visitors don’t.
Also with people like Google pushing FF, that number is going to keep on falling.
Tell them that 20% of your visitors use FF, so since their systems are garbage, you want 120% payout to help account for this. 
How does anyone “professional” design or program something that’s not friendly in other browsers besides IE? That’s amazing.