Forssell gave the Blues a two-goal cushion just before half-time
Alex McLeish secured his first home win as Birmingham boss as the Blues saw off an ineffective Boro at St Andrews.
They went ahead when Stewart Downing sliced Gary McSheffrey's free-kick into his own net inside the six-yard box.
On the stroke of half-time, Mikael Forssell made it two when he prodded in a corner from close range after Cameron Jerome had flicked on.
Boro had plenty of the play but lacked composure and McSheffrey added an injury-time penalty after being fouled.
Aided by a pulsating public address system, the home fans generated a lively atmosphere in the build-up to the match, but it was Boro who had the first chance when Jeremie Aliadiere fired straight at Maik Taylor.
Boro's top scorer Downing soon got himself on the scoresheet again, but unfortunately for Gareth Southgate it was at the wrong end and was the first of two critical defensive lapses from the visitors.
McSheffrey's free-kick from the right wing fizzed across goal and in attempting to clear, Downing merely succeeded in shinning past his own keeper.
Boro continued to threaten on the break, with Aliadiere and Tuncay offering plenty of pace and promise, but lacked a final ball of quality and precision.
Stephen Kelly needed to clear off the line after Taylor failed to gain distance on his punch, while Tuncay headed straight at the keeper when a yard either side would have given his team an equaliser.
That proved costly as in the dying moments of the first-half Forssell poached his team's second goal, prodding in from two yards out after Jerome rose clear at the near post to flick on Larsson's left-wing corner.
Boro needed something special in the second half but could not find it as Tuncay volleyed over after gaining space in the area.
McSheffrey calmly sends Schwarzer the wrong way for the third
Birmingham continued to look the more incisive in front of goal and Larsson was narrowly wide of the left-hand upright with a shot on the run from 20 yards.
The determined Tuncay tested Taylor with a swerving shot from 25 yards that the keeper was happy to tip over the bar.
The Turkish international skipped through again but was denied by Kelly's sliding challenge.
Jerome had an opportunity to give the scoreline a rather flattering look but he shanked embarrassingly wide with his left foot and remains without a goal at St Andrews this season.
Referee Mark Halsey then presented the home side with another chance to get the third when he adjudged that Robert Huth had felled McSheffrey, who duly sent Mark Schwarzer the wrong way from the spot.
Birmingham boss Alex McLeish:
"They were hurt by the performance last week (3-0 defeat at Bolton).
"Their response was important and the players proved they can bounce back so can take a lot of heart from that."
Middlesbrough manager Gareth Southgate:
I don't think we played well enough and didn't deserve anything. "We created a few openings but you cannot expect to come to a play like Birmingham and not compete. "Anybody who thought we had turned the corner was wrong and we have a lot of hard work to do."
Birmingham: Maik Taylor, Kelly, Jaidi, Ridgewell, Queudrue, McSheffrey, Muamba, Johnson, Larsson (Nafti 82), Jerome, Forssell (O'Connor 77).
Subs Not Used: Doyle, Djourou, De Ridder.
Goals: Downing 22 og, Forssell 45, McSheffrey 90 pen.
Middlesbrough: Schwarzer, Young, Wheater (Huth 46), Woodgate, Pogatetz, Boateng, Rochemback, Arca (Johnson 70), Downing, Sanli, Aliadiere (Lee 46).
Subs Not Used: Turnbull, Shawky.
Booked: Arca, Woodgate.
Ref: Mark Halsey (Lancashire).
BBC Sport Player Rater man of the match: Birmingham's Mikael Forssell 7.47 (on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.