first和firstly在作副词用的时候,有何区别?

当first和firstly作副词用的时侯,如果“用于分开并依次列举所述各点”时,是没有什么区别的,可以替换.e.g.First(ly),I'll mention the advantages,then I'll mention the disadvantages.
但当用于修饰行为动词时,则只能用first.
e.g.I must ask my parents first
When did you first go to the country?