Songfacts®:

• 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."
• Running 9:36, most of this song is taken up by instrumental passages, but there are some lyrics, which seem to be set in medieval times and describe a journey of some kind. Peter Gabriel sang lead, but Tony Banks and Mike Rutherford this particular lyric. "It was just following the idea of a river and then I got a bit caught up in the cosmos and I don't quite know where I ended up, actually," Band told us. "But, it just about stands up, I think, for the song. For me, musically, it's got two or three really strong moments in it and fortunately they really carried us along. It's become one of the Genesis classics and I'm very happy for that."
• A live favorite, the band played this song in concert throughout their career, with Phil Collins handling vocals after Peter Gabriel left in 1975.
• The piano intro that leads off this track is some of Tony Banks' most acclaimed work. Recording it was a challenge, as noise from the pedal kept bleeding onto the track.
  
  It was common for the band to write in bits and pieces, then combine them to make fully formed songs, which is what happened here. "I had this sort of arpeggio idea that I was working with," Banks said. "I'd written another piece which used a similar feel, which we never ended up using, and I just had this section of it, which I then developed and made this piece of. I thought it worked really well as a piano piece on its own, and then it worked well with an arrangement, as well. So, it's just one of those things. With Genesis, we just did what appealed to us, really. We didn't worry too much how other people were going to respond to it."
• Pianos are not easy to transport and take up a lot of room on stage, so Tony Banks had to use an electric piano to perform this live, which never sounded as good as the real thing.