If you knew that 90% of your visitors would be using IE you should have developed with it, or at least test with it sooner in your development cycle .
Aside from that, you specified a left pane of width="200". Your "What's new" box is exactly 200 pixels, what you specified, look for dnn_LeftPane. It streches to about 233 pixels in Firefox, to match the bottom box, which by the way, cannot be 200 pixels because the email address you show there, in the font you are using, pushes the right side out in order to fit.
As far as a solution, I saw that you specified 100% in your "containermaster_blue" class and that is not doing it in IE. Is that truly incorrect? The container is 100% inside of the left pane which you specified as 200px. Why is Firefox expanding it to match the bottom box I don't know. Is it correct for FF to assume you want the top table to match the bottom table? I think your options are to expand the size of the left pane to about 230px or to reduce the font of the email address in the bottom box.
I don't want to start a philosophical debate here, and I don't know your reason to use Firefox as your primary browser, but as a developer or Web designer knowing that the great majority of users use IE you have to get it into your test cycle very (very) early on.
Carlos