When IE7 came out, I personally made the switch from IE to FF. The reason is that I like to play online flash games. In IE7, they made the decision to make the up and down arrows scroll the window up and down regardless if the user has clicked on the flash media or not. This is a break from IE6 and every other browser I have used.
This hasn't mattered until I started my own online game website. It was easy for me to solve the problem personally because I just made the switch from IE7 to FF. However, I'm not sure most people that use my website know to make that switch and they might get annoyed and go somewhere else.
Thanks in advance.