How can I make event.srcElement work in Firefox and what does it mean?

srcElement is proprietary property originally coming from IE. The standardized property is target:

var target = event.target || event.srcElement;

if(target.onclick == null) { // shorter than getAttribute('onclick')
    //...
    document.mainForm.submit();
}

Also have a look at quirksmode.org - Event properties for more cross browser information.

Regarding the question what it is doing:

event.target / event.srcElement contains a reference to the element the event was raised on. getAttribute('onclick') == null checks whether a click event handler is assigned to element via inline event handling.

Is it important? We cannot say because we don't know what the ...code.. is doing.

In IE the event object is available in the window object already; in Firefox, it's passed as a parameter in the event handler.

Example

JavaScript:

function toDoOnKeyDown(evt)

{

    //if window.event is equivalent as if thie browser is IE then the event object is in window
    //object and if the browser is FireFox then use the Argument evt

    var myEvent = ((window.event)?(event):(evt));
    //get the Element which this event is all about 

    var Element = ((window.event)?(event.srcElement):(evt.currentTarget));
    //To Do -->

}

HTML:

<input type="text" id="txt_Name" onkeydown="toDoOnKeyDown(event);"/>

As you notice when we called the function inside the html we have added a parameter event just in case the browser is Firefox.

I have read in an article that the event object in IE is called window.event and in Firefox we have to put it as a parameter.

In case you need it to be attached in the code:

document.getElementById('txt_Name').onkeydown = function(evt) {
    var myEvent = ((window.event)?(window.event):(evt));


    // get the Element which this event is all about 

    var Element = ((window.event)?(event.srcElement):(evt.currentTarget));
    // To Do -->
};