It probably has more to do with your DNS server and its timeout, unfortunately. I can't seem to find anything to support this at the moment, but I remember that if BIND times out on getting a response from a remote DNS server, it sends a response to its client that the hostname does not exist. Meanwhile, it finally receives and caches the information about the initially requested hostname, so that when you try again, it immediately sends the correct response. It might be possible to configure Firefox to ask the DNS server more times than it currently does, but I'm not aware of any such option.

Edit: Okay, I found it now. BIND 8 and earlier fail to do a "query restart" when they encounter a glueless record during a recursive lookup. That is, if they look to see the correct nameserver for your request, and the response they get for that has that nameserver's name but not IP address, then it drops the initial query and tries to look up the nameserver's IP. That means that your client has to ask again, and this is probably what you're seeing. The DNS server never responds to your initial request, so your browser has to ask again.