Ed has just replied with the fix for this over at themeforest. I’m copying it for anyone else having the same issue.
It has fixed it for me 🙂
Thanks for the link. This appears to only be an issue in firefox. We’ll fix this for the next update. For now, edit /js/functions.js and remove lines 431-433:
jQuery('.variations_form').find('select').each( function() {
jQuery(this).customSelect({customClass:'woo-select'});
});