The 27-year-old is the latest departure from the Blues following the expiration of her contract.
Staniforth has been a top level player since breaking through with Sunderland at the age of 16. In 2009, she played a pivotal role in her hometown club in winning the Women's Premier League Northern Division and reach a remarkable FA Cup final. Despite losing the final to Arsenal, Staniforth immediately stood out as a bright talent.
She made her Women's Super League bow when signing for Lincoln Ladies in 2010 and has gone on to make over 100 appearances at the top level. She reached a second FA Cup final of her career in 2012 with Bristol City, picking up a major injury which kept her out of action for a year.
She returned to Sunderland in 2016 after an injury-hit spell with Liverpool, causing her to miss the Reds' title winning 2014 season. Her fitness and form recovered back in the North East, being named captain in 2017 and becoming a fans favourite. Her form attracted the interest of Birmingham City, joining the Midlands clubs in 2018.
She was finally given her England debut in 2018 for an World Cup Qualifier against Kazakhstan. Later in 2019, she formed part of England's victorious SheBelieves Cup campaign as well as Phil Neville's World Cup squad. She has made 16 appearances for the Lionesses in total, scoring twice.
The loss of Staniforth on a free is the latest in a long line of high profile departures from the Blues.
Following a difficult 2019/20 WSL season, much of their squad has been lost to their top-flight rivals. Other departures include captain Kerys Harrop, fan favourite striker Rachel Williams and Scottish international Chloe Arthur.
In total, the Blues have released seven first team players on frees as well as losing Georgia Brougham after her loan from Everton expired.