You might notice that multiplayer games in Firefox seem laggy compared to Chrome on the same machine/connection. We reported the issue to Mozilla and thanks to some contributions from other developers they've identified the issue and fixed it for Firefox 29:
The problem is Firefox currently buffers messages sent over WebRTC DataChannels. A sent message might be deliberately held back until it has more data to send, or receives acknowledgement from the other end (look up "Nagle's algorithm" for more). This works badly for multiplayer games since data needs to be sent with the minimal latency possible and messages could be held up locally for an extra 50-100ms.
Firefox 29 is apparently due out in April, and hopefully multiplayer games should be less laggy in Firefox from then!