I've worked at a couple of different places where users were color blind to blue and another to red and having a quick and easy way to be able to change the color of unread mail would be greatly appreciated by those people who can't tell the difference between blue and black. Bold and not bold isn't as easy to spot when looking at an inbox full of emails, and requiring someone to create a userchrome file buried in the AppData folder is probably a little more than most users are willing to do...