The 29-year-old is Chelsea's leading women's goalscorer, having scored 110 times in 175 appearances.
Kirby, who joined the club from Reading in 2015, signed a two-year deal in 2021 with the option of a further 12 months.
She said at the start of May that she would miss the World Cup with a knee injury sustained in February during a League Cup semi-final against West Ham.
