This code works in Google Chrome and IE but not in FireFox.

function drawChildren(names) {
    var c = document.getElementById("myCanvas");
    var ctx = c.getContext("2d");       
    var ctxcolor = "#000000";

    if(listeners < 1){
        c.addEventListener("click", getPosition, false);
        listeners = 1;
    } // At the click function "getPosition" is executed
}

function getPosition(event) {

    if ( event.offsetX == null ) { // Firefox
        clickedX = event.layerX;
        clickedY = event.layerY;
    } else {                       // Other browsers
        clickedX = event.offsetX;
        clickedY = event.offsetY;
    }

    checkCoordinates(clickedX, clickedY);  
}