There is something broken with your site, it looks like it is due to HTML comments. I did a view source, and for one thing I see the shadowbox initialization code twice, not sure what is causing that.
However this appears to be the bit that is causing shadowbox to not be defined:
<!--<div id="footer">
<script type='text/javascript' src='https://yui.yahooapis.com/2.8.0/build/yahoo-dom-event/yahoo-dom-event.js?ver=2.8.0'></script>
<script type='text/javascript' src='https://www.latoileculturelle.com/lesite/wp-content/uploads/shadowbox-js/aac33fafb71b1cec21a231278c13c215.js?ver=3.0.3'></script>
<div id="mycontactform"> </div><script src="https://www.latoileculturelle.com/lesite/wp-content/plugins/tweetmeme/button.js" type="text/javascript"></script>
<!-- Begin Shadowbox JS -->
You have started an html comment <!-- directly before <div id="footer">, however you tried to put the close tag after the call to <?php wp_footer(); ?>. Which means that it is picking up the close tag --> that appears after Begin Shadowbox JS, and ignoring the --> that happens after </html>.
This is causing browsers to ignore the script tags that appear between that footer div and the shadowbox initialization code.
If I take a guess at what you were trying to do it is move the footer section to another spot in the page. Keep in mind that the <?php wp_footer(); ?> call should go directly before </body> and not elsewhere. Try removing the wp_footer code from earlier in the code and moving it to the place where I suggested.
Or you could just clean up your code i general and remove everything that was commented out using html comments, and then place the wp_footer call in the appropriate place.
