Actually, the TD works quite well under IE9 (as long as you're using 2.5.12). There were only 3 or 4 real issues that I am aware of, and I fixed them all in 2.5.12.
One issue was a flickering if you were using an image for a background on the Game window. Honestly this is an IE9 issue, but I was able to work around it in the TD. There's still a very minor flicker that can occur when the screen changes, but as I've heard not a peep about it since 2.5.12 was released, it might be that I'm the only one that even notices it. Totally fixing the issue would have meant a lot of code change, and I decided to put that effort into the new version.