Banks wrote most of "Firth of Fifth" on his own, and had presented it to the group for "Foxtrot", but it was rejected. He reworked some sections of the song for "Selling England by the Pound", where it drew a more positive reception. The track opens with a solo piece for piano, that is repeated by the band later in the song. Banks recalled the difficulty to remove the noise created by the piano pedal in the studio, so he played the passage without it which he found difficult. Hackett took one of Banks' piano figures and rearranged it as a guitar solo, which dominates the latter part of the track. Banks later deemed the lyrics, to which he contributed with assistance from Rutherford, as one of the worst he had worked on. He had aimed to follow "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".