You are the victim of an IE bug, I guess.
In the article “Cinco de Mayo Showdown” you have an image, which has a greater width then the <div>-element #mainCol, which contains the article previews. FF does not resize widths and heights (as it should be), if elements like uninterrupted text or images don’t fit in, but IE shows that misbehaviour.
Your solution is to add “overflow: hidden;” to #mainCol in your css.