negative space causing horizontal scroll bar.

I’ve read forum on this with no avail. It is happening in FireFox 9.0.1. The blank space to the right of my page causes a horizontal scroll bar over to lala land. I think it is caused by the negative margin in the original twenty-eleven css I have been working off of. But I can’t figure out how to adjust the container and side content so that they both still display correctly. Here is a screenshot to what I am referring to: https://washingtoncreekdesign.com/forums/screenshot2012-02-14.png

The website is wintersynergy.com. I have modified Twenty Eleven CSS, so please check it out. Here are a few bits that may be relevant. I’m not sure if there more to it this though.

#container {
	float: left;
	margin: 0 -293px 0 0;
	width: 100%;
}
#content {
	margin: 0 310px 0 0px;
}
#sidecontent {
        width: 293px;
	float: right;
	overflow: hidden;
}