Rookie Rodney Wallace earned the penalty when he was taken down in the penalty area and Moreno steered home his shot in the 42nd minte.
Coach Tom Soehn left Santino Quaranta, Ben Olsen and Fred off the game-day roster while making eight changes to the starting XI that played San Jose in a 2-2 tie Saturday. Recently acquired midfielder Danny Szetela went the full 90 minutes in his first United start, and Ely Allen - signed as a free agent over the weekend -- dressed for the match but didn't play.
Janicki lost the ball to Benitez a few yards outside the D.C. penalty area and the quick Salvadoran striker punished the mistake ruthlessly by firing a low shot past goalie Milos Kocic just inside the far post.
A few minutes later Christian Sanchez eluded a pair of challengers to belt a shot that Kocic saved. Soehn used his first sub in the 40th minute, pulling off ineffective forward Ange N'Silu and sending on Luciano Emilio, and after just two more minutes United tied the game when Sanchez decked Wallace as he chased a through ball, and Moreno tucked the ball inside the post despite Gomez's dive in that direction.
Soehn brought on Christian Gomez for the second half and the United attack livened up. Moreno hit a cross that Emilio drove inches wide of the near post, and took a pair of errant shots himself before making way for Chris Pontius.
A powerful shot by Dennis Alas stung Kocic's gloves as it squeezed through them toward the goal but the keeper regained possession just in time. Pontius swung a cross to the far post that Wallace hit first-time but the Firpo defense blocked the shot.
United outshot Firpo, 20-16, and nine shots required saving by Gomez. At the other end, Kocic saved four. With the away goal, Firpo can advance by tying, 0-0, in the second leg Tuesday in San Salvador. United needs to win, or tie while scoring more than one goal.
The tie marked the seventh straight match in the Concacaf Champions League that United has failed to win. It lost five games and tied one in group play of the 2008-09 competition.
July 28 in Washington, D.C.
D.C. United 1, Luis Angel Firpo 1.
Goals: Benitez 26; Moreno (penalty kick) 42.
D.C. United - Kocic; Janicki, Jakovic (Gomez, 46), John, McTavish, Wallace, Jacobson, DiRaimondo, Szetela, N'Silu (Emilio, 40), Moreno (Pontius, 66).
Luis Angel Firpo - Gomez; Turcio, Monteagudo (Quijada, 88), Alas, Campos, Sanchez, Salazar, Leguizamon, Franco (Merino, 70), Benitez (Barrera, 79).
Referee: Paul Delgadillo.