This is, as stated by others, an issue with the new strict dtd enforced by IE 8 in default mode. switching to "Compatablity View" does indeed resolve the problem, but I would argue, is not a good long term solution - forcing your users to configure their browsers.
That said, I am writing this using IE8 in default mode and I'm guessing, this is the latest FTB control, so something has been fixed.
Anyway, you are using a FREE control. Yes, you are within your right to modify it, but not to complain that the free control you are using has been modified to address evoloution. It is but a small price to pay for the tons of work done by others for your benefit. Still, I would argue that you should never have modified the source in the first place. The preferred method imo, would be to use the control as a base class for any custom modifications that you need for your application. That way, when the makers of your free control release an update, you simply plug it into your project and go. No merging of custom changes needed.
Those are my thoughts