I have seen this as well -- when I have Charles Debugging Proxy running. This is a new error for me, as it has never been a problem in the past. But when I close Charles, everything works fine.
I have been having significant problems lately in Charles with https and FF. Since the latest versions of FF, I can't get the SSL proxying to work correctly and sometimes sites won't even load properly. I am not having any problems with Charles in Chrome or IE.
Do you have a debugger of some sort running?
