All other browsers work perfectly fine. However, when firefox tries to execute this code:

if (!e) var e = window.event;

e.cancelBubble = true;

if (e.stopPropagation) e.stopPropagation();

it crashes and the console displays the following error:
TypeError: e is undefined

edit 1:

function clickInactiveTab() {

    $(this).attr({class: activeTab});



    $(.inactiveTab).hide();

}



function clickX() {

    $(this).parent().attr({class: inactiveTab});

    $(.inactiveTab).show();



    if (!e) var e = window.event;

    e.cancelBubble = true;

    if (e.stopPropagation) e.stopPropagation();

}

What it does it, change the style of a div when clicked and hides all other divs from the class. When someone clicks the x that's inside the div, it should change the style back and show the hidden divs.