Display suddenly not rendering CSS in Mozilla

We are suddenly having a very strange problem with the page not displaying properly (the CSS is not rendering) in Mozilla, but seems OK in IE.

The site has been working fine in Mozilla for the past several months, and we haven’t changed anything with the theme recently.

The odd thing is that if you hit “refresh” the page renders OK. But if you goto any other page you get the same display problems. It’s not a cache problem though, because if you go back to the first page you refreshed, it displays wrong again (and I’ve cleared the cache).

Any ideas? You can see site here: https://www.routesetter.com

Thanks!