The title doesn't appear in the lyric, which is not unusual for Genesis. In this case, it has little to do with the song, but is something the band found amusing. In Scotland, there is a river called the Forth, and "firth" is a term meaning an inlet or estuary. Thus, the Firth of Forth. Tony Banks named the track "Firth Of Fifth" as a play on this.
"We're talking about the early '70s here, so it was a little bit pretentious," he said in a Songfacts interview
. "It sounds more profound, in a way, than it is because it was supposed to be just a slight joke, really, as a title."