Build FF17 with --disable-webrtc on all platforms
It looks like FF17 ships with and builds WebRTC. I'm not sure how much of the WebRTC is actually exposed, but the code definitely has the ability to create UDP sockets independent of proxy settings if it is used. I also don't see any about:config entries for it.
We want to disable this by compile option.
To upload designs, you'll need to enable LFS and have admin enable hashed storage. More information