Let’s face it- old browsers are a pain in the rear. Browsers like FireFox 2 and Internet Explorer 6 do not behave like modern browsers. They do not render HTML in the same way and do not interpret JavaScript in the same way. You can, most of the time, fix these issues, but the process costs a lot in terms of testing, development, and time to market. I was just involved in a project were a client insistent on going through 12 browsers and paid hundreds of thousands of dollars for that line item.
The problem is that clients still use these browsers and expect to view your site properly with their legacy browsers. So, when do you stop supporting old browsers?