Firefox bug - TBB queries the A record of the hostname of the machine it is running on.
When Firefox starts, it does a DNS query for the A record of the hostname of the machine it is running on.
If that hostname resolves to a public IP address, then Firefox will have information that could be used to deanonymize the user, and it is unclear what Firefox is doing with that information or why it needs to collect it.
This query is not done via Tor, it uses the system resolver, even when using TBB. This was discovered by configuring BIND to log queries and running tail -f on the query log while starting TBB Firefox and regular Firefox, which each exhibit this behavior shortly after starting.
Some quick google searching seems to indicate that I am not the only person who has noticed this. Many people seem to have trouble getting Firefox to start if their machine's hostname does not resolve. I found one bug in the Mozilla bug tracker (sorry, I can't find it back again to note the number), where someone was complaining of exactly this, but the response was a denial that the behavior was happening.
This should be looked into and it should be confirmed that nothing nefarious or negligent is being done with the result of that DNS query.