Essentially, one of the threads in the list is one long word with no spaces. Firefox is unable to wrap the thread title across multiple lines since it's all one word. Therefore, instead of stretching out the table and ruining the layout of the site, it chops it off where the table is supposed to finish.
I had to do it this way because without chopping the table down when a long thread title pushed it out, Firefox extended the table underneath the sidebar, but IE simply pushed the sidebar down to the bottom of the page making it unusable. Therefore, for the sake of browser consistency under this rare circumstance, I have opted to just chop the table :)