The Seagulls' skipper scored 12 goals by mid-December but he has dried up since then.
Forster could have been forgiven for anxiety when Orient midfielder Adam Chambers needlessly handled inside his own area.
But Forster showed nerves of steel to send keeper Glenn Morris the wrong way from the spot and extend Brighton's unbeaten home run in League One to seven matches.
It was no more than they deserved after Jabo Ibehre threatened to give Orient a flattering win with ten minutes left.
Ibehre, who turned down a move to Brighton 18 months ago, made amends for a glaring miss a few minutes earlier when he guided in a header from a cross by Jason Demetriou.
Brighton looked like being punished for several missed opportunities in the first half.
Forster's strike partner, Glenn Murray, was the main culprit, he had scored four times in as many home games since a £300,000 move from Rochdale in January, but he spurned several chances to put Brighton in command.
Murray headed over and straight at Morris from crosses by Dean Cox.
He was also clean through from a misplaced pass by Sean Thornton, but was forced wide as he rounded Morris and Cox's subsequent shot was parried by the keeper for a corner.
Murray was not the only culprit for the home side. Dean Bowditch, on-loan from Ipswich for the second time, lobbed over when well placed.
Forster rounded Murray from Bowditch's slide-rule pass just before the break, only for his effort from the tight angle to be blocked on the line by Sam Oji.
Orient came into the game more in the second half, but Murray again should have punished them in the 63rd minute when he dispossessed Oji inside the box.
He evaded Morris but ran out of room and opted to feed Forster, who lashed wide.
Ibehre punished Brighton for their profligacy, but Forster's cool head saved them and they could have snatched victory deep into stoppage time.
Tommy Elphick's header from a corner by substitute Jake Robinson was cleared off the line by Paul Terry and Morris held on to French sub David Martot's follow-up shot through a crowd of players.