Simple problem totally defeat noob

This should be quite simple but I am at a loss.

I have several functions in a Flash interactive that fire in response to buttons being clicked or rolled over:

function firstFunction(evt:MouseEvent):void{
someThing.happens.right.now;
}
firstButton.addEventListener(MouseEvent.ROLL_OVER,firstFunction);

function secondFunction(evt:MouseEvent):void{
someThingElse.happens.right.now;
}
secondButton.addEventListener(MouseEvent.ROLL_OVER,secondFunction);

What I would like to do in addition to the button events is have, say, the first function open when the page loads. Of course when I do this:

firstfunction();
function firstFunction(evt:MouseEvent):void{
someThing.happens.right.now;
}
firstButton.addEventListener(MouseEvent.ROLL_OVER,firstFunction);

function secondFunction(evt:MouseEvent):void{
someThingElse.happens.right.now;
}
secondButton.addEventListener(MouseEvent.ROLL_OVER,secondFunction);

Flash helpfully explains that it’s expecting and argument when I call the function without the button event.

I’ve tried this:

firstfunction(null);
function firstFunction(evt:MouseEvent):void{
someThing.happens.right.now;
}
firstButton.addEventListener(MouseEvent.ROLL_OVER,firstFunction);

function secondFunction(evt:MouseEvent):void{
someThingElse.happens.right.now;
}
secondButton.addEventListener(MouseEvent.ROLL_OVER,secondFunction);

And sometimes it works and sometimes it doesn’t.

Any thoughts?