Firhill became designated under the Safety of Sports Grounds Act in 1977, which reduced its capacity from over 40,000 to 20,500. In 1986, Firhill became the first Scottish ground in modern times to be used by more than one team, when Clyde moved in after being evicted from Shawfield. This arrangement lasted until 1991. Hamilton Academical then shared Firhill with Thistle in two different spells, the first arrangement beginning in 1994. To cope with the resulting additional usage, undersoil heating was installed in 1994.