ユーザー登録のシーン

signInWithPasswordと、signUpの違い
登録では、signUpがただしかった模様

❌

.post(
          "/accounts:signInWithPassword?key=hogehogehogehogehoge",
          {
            email: this.email,
            password: this.password,
            returnSecureToken: true,
          }
        )

⭕

.post("/accounts:signUp?key=hogehogehogehogehoge", {
          email: this.email,
          password: this.password,
          returnSecureToken: true,
        })

https://firebase.google.com/docs/reference/rest/auth