Post breaks sidebar

I have a blog aggregator site and just noticed that one of the posts is breaking my sidebar in Firefox (but not IE). I don’t know much about css or coding problems like this. I was hoping someone might be willing to take a look at the site & give me a sense of which post is the offending one. Or whether perhaps there’s something in my css that’s allowing/causing this to happen.

My other problem is that since the site uses the WP Feedreader plugin which updates via rss feeds, once I DO find the culprit I won’t just be able to delete it since the feed update will merely republish the post. Any ideas on what to do would be much appreciated.