Referee Chris Sarginson threatened to abandon the game after a series of missiles were thrown at the assistant referee and Boro goalkeeper Jason Steele.
However, the FA have opted not to charge Millwall with failing to control their fans.
Lions chief executive Andy Ambler said: "The FA have taken into account the arrests, and the steps Millwall FC have (implemented), and continue to implement, to stop similar incidents occurring in the future.
"The FA have informed us that they will monitor every game between now and the end of the season. They have advised us that they can take retrospective disciplinary action should there be any further incidents."
Source: Northern Echo
Source: Northern Echo