Complex sentence: It appeared for the North American races at the end of the 1974 season and was gradually developed through 1975, although Firestone's decision to quit racing meant that a major partner was lost.
Split into simpler sentences: It appeared for the North American races at the end of the 1974 season and was gradually developed through 1975. However, Firestone's decision to quit racing at the end of 1974 meant that a major partner was lost.