I’m no programmer, but it looks a bit odd to me that in the code, you are using two stylesheets directly after eachother.
I don’t know the theme, so I don’t know how it’s built, but you are using Hemingway/style.css and Hemingway/styles/white.css
I would try to get rid of one of the two. <typo>
Think we can safely rule out server problem at this time since other browsers are rendering it. However… Just looked at your validation and you have an extra div closing issue. THAT would definitely cause problems. See if you can track down and fix that guy and I’ll bet it will work in FF.
The odd thing is that if I use the FF developer’s plugin to load/edit your CSS (without changing anything), suddenly your theme shows fine in Firefox. Maybe that’s a clue or a hint, but I’m not sure what to suggest next.