ReferenceError: event is not defined error in Firefox

I've made a page for a client and I initially was working in Chrome and forgot to check if it was working in Firefox. Now, I have a big problem because the whole page is based upon a script that doesn't work in Firefox.

It's based on all "links" that have a rel that leads to hiding and showing the right page. I don't understand why this isn't working in Firefox.

For instance pages have the id #menuPage, #aboutPage and so on. All links have this code:

<a class="menuOption" rel='#homePage' href="#">Velkommen</a> 

It's working perfectly in Chrome and Safari.

Here is the code:

$(document).ready(function(){

//Main Navigation


$('.menuOption').click(function(){

  event.preventDefault();
  var categories = $(this).attr('rel');
  $('.pages').hide();
  $(categories).fadeIn();


});

// HIDES and showes the right starting menu
    $('.all').hide();
    $('.pizza').show();


// Hides and shows using rel tags in the buttons    
    $('.menyCat').click(function(event){
        event.preventDefault();
        var categori = $(this).attr('rel');
        $('.all').hide();
        $(categori).fadeIn();
        $('html,body').scrollTo(0, categori);

    });


});