It might help to post more lines from the log.

Maybe it's some broken .js based browser-detection web-hackery. Does the problem still persist if you change Firefox's user agent setting?

Stretching a bit... since it's being blocked by the router, but only happens with Firefox, maybe Firefox is communicating with the server in a different fashion than Explorer or Chrome, and opening multiple connections. Thinking about how many ways iptables can be configured, perhaps the firewall is blocking the packets based on connection state.