In an interview with co-star Stanley Tucci in the December issue of Attitude, out now, the star adds the question is "still alive."
The actors play a gay couple on a road trip of the Lake District in the tearjerker, out soon; Firth has previously played gay in 2010's A Single Man.
"It's something I take really seriously"
"I don't have a final position on this," Firth said. "I think the question is still alive. It's something I take really seriously, and I gave it a lot of thought before doing this."
Tucci and Firth in Supernova (Pictures: StudioCanal Bleecker Street)
The Oscar-winning star of The King' Speech added: "Whenever I take on anything, I think it's an insufferable presumption. I don't really feel I have the right to play the character. That's always the starting point. What do I know about this person's life?
"How can I presume to set foot in this person's lived experience, let alone try to represent it?"
Tucci, who played a gay character in 2006's The Devil Wears Prada, added: "For so many years, gay men and women have had to hide their homosexuality in showbusiness to get the roles they wanted - that's the problem here.
"Anybody should be able to play any role that they want to play - that's the whole point of acting."
In an interview this week, Lord of the Rings star Viggo Mortensen defended playing a gay man in his self-directed movie Falling.
Supernova explores the onset of dementia through one of its characters, and has been applauded for its sensitivity. It currently holds a 90% certified fresh score on Rotten Tomatoes.