Mozilla firefox not working with window.onbeforeunload

I'm using window.onbeforeunload to display a message to the user on windows close, the function works well with Chrome and IE but it doesn't work with Firefox, I'm using Firefox version 26.0 I have tried many but with no mean, somebody said that its a bug in Firefox as in this post and another suggests some solutions as in this post I tried all the solutions available using Javascript and jQuery but it doesn't work, now I display a confirm dialog but the browser default dialog appears after it and I'm not satisfied with that, I tried also to prevent the browser default dialog from appearing using preventDefault() but also with no mean! if there's any solution to this problem it will be great, here's how I used the window.onbeforeunload:

<script>
window.onbeforeunload = confirmWinClose();
function confirmWinClose() {
     var myVar ='${isFireFox}';
     if(myVar=='true'){
         return confirm(confirmExamClose);
     }else{
         return confirmExamClose;
     }

}
<script>

Note:isFireFox is a jsp variable that I used to know the type of the browser using User-Agent Header and confirmExamClose is the message that I display to the user.