I see you are using the class .contactform-button
Is this the class name on the button or a name you created? Just trying to clarify the issue.
You can see what class they have assigned to the button by using your browsers inspect option.
I had the same issue in FF, IE and Chrome (different messages) This post actually corrected the issue in all browsers for me!
http://www.kimwoodbridge.com/wordpress-flash-uploader-fix-for-the-http-error/
