WP theme Layout problems, firefox vs explorer

[GrosCochon]

Hi people,
I just noticed there's a serious layout problem with my new theme, could be the reason why my traffic has been so low for a few days....

The site looks great in firefox, but in explorer my left sidebar and my main post column are overlapping, how can I solve this? I'm guessing it's something in CSS but I dont have much knowledge about it.

I know it's probably a lot of work but if someone could point me in the right direction I should be able to do the rest with css tutorials..

here's my link: http://www.latinateensblog.com

Thanks