Hi,
It seems the problem is linked to FireFox behaving differently because I didn't had the problem with Chrome on your website.
Also, it seems to be linked to the customization mechanism because if I click on the "reset to default view" button at the bottom of the product edit form, it worked again until I saved the product again.
What you can do is to go in your PHPMyAdmin and open the table hikashop_config. In it, add an new entry with the namekey form_custom and the value 0
It will deactivate the customization mechanism and hopefully circumvent the problem.