Firth starred in the first two films alongside Renee Zellweger and Hugh Grant, and it was recently reported the cast will be reprising their roles for another picture.
The British actor reveals he would like to see the writers play around with the story, by pushing the film forward by several decades.
He tells U.K. radio station Absolute Radio, "I think we're all getting older now and the only circumstances in which it might be fun are to see us all having physically fallen apart. Just with a gut and no hair and Hugh Grant being a sad, tawdry old Lothario. It's all too easy to imagine, really."