Update: The JAL 787 has landed safely in Boston. Apparently a fuel pump warning was the reason for the return.
Earlier: Data from Flightaware shows the 787 is in a holding pattern at 15,000 feet over the Atlantic and flying at 290 knots, a situation that would not be consistent with a major inflight emergency such as an onboard fire. In that case, the crew would have declared "mayday" and would have been given a direct approach to the airport in order to land as soon as possible.In case of fire or other potentially disastrous event the crew would also have elected to land at an alternate airport along the way. Choosing to return to Boston indicates most likely a less-serious emergency. The original story begins here: A Japan Airlines Boeing 787 en route from Boston to Tokyo has made a U-turn over Canada and is returning to Boston, late on Thursday.Source: Alberto Riva
Gravity always wins!