According to animesenpai.net…In the last chapter of the Fire Force manga, we saw the appearance of Excalibur, who is a well known character from the Soul Eater universe, and Excalibur is 800 years old in the Soul Eater manga. We also saw glimpses of the first witch in the bonus panels. Soul Eater series main character, Maka also makes an appearance in the final chapter of Fire Force. It was confirmed that Fire Force is a prequel story to the Soul Eater, which takes place in the same world a long time ago. Also the existence of the weird moon and sun gave fans a major hint in the first chapters as well.
FTO: NERD TALK 