All,
From what we found in the logs - it's not a Firefox problem. ( And I'm writing this from Firefox 21.0 ) - we ( the hosting center ) recently deployed an updated security patch - which for some reason worked fine on all but 1 of the WebServers in the SH cluster. This would have caused a condition where any user routed to that specific server would have seen a "Parallel's Hsphere Error page".
If that is what you saw - then we are aware of - and fixed that problem late last night.
-Michael