The actor plays gay college professor George Falconer in the Tom Ford-directed adaptation of Christopher Isherwood novel A Single Man.
Firth told Parade: "If you're a straight actor who takes on the role of a gay man, it's obviously not the same as being one in this business. I think there are still some constraints against being openly gay.
"It is to do with whatever sexual taboos are in place. In Hollywood, I know that some actors are far more circumspect about keeping that secret hidden. There are not a lot of openly gay leading men."
He added of his recent role: "There's sex running all the way through the movie, even though we don't need to see anybody doing it.
"I think the fact that the man I play is comfortably open about the fact that he is gay is definitely significant. But I think it just reinforces the power of the story, which is about a love between men that has a very accessible, universal quality."
Openly gay star Rupert Everett recently claimed that young homosexual actors should not come out because it will harm their careers.