我觉得在你说的这种情况下first和firstly是可以等同的.是不是没有和后面段落里的secondly, thirdly对应起来?
first第一,首先.除了用于开场白,还有第一的意思
firstly,形容词,强调第一个（做完的）,最先的
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.
I must ask my parents first
When did you first go to the country?