I've actually had this problem for a while now but just decided to troubleshoot it. So here's what I was initially seeing:

Then i found and disabled a single CSS rule and got this:

I would file this issue with the rust-lang/rust-www repository but then I had the same problem with viewing documentation pages from 3rd-party crates hosted on Github so I'm assuming it's Rust's documentation generator itself that is generating this CSS rule. I have installed the "fira" font from the ArchLinux AUR and no longer have to disable the CSS rule, but there really should be a fall-back font set. In case it's relevant I'm running ArchLinux x86_64, Gnome 3.14.2 and Firefox 33.1.1.