Issue with grouped checkboxes in firefox

Can someone comfirm this please. I'm using:

OSX 10.7.5
Firefox 33.1.1

1) Go to either

http://djangular.aws.awesto.com/form_validation/

or

http://djangular.aws.awesto.com/combined_validation/

2) Scroll to 'notify me' and select 'postcard' -- i get the 'required' error message

3) deselect 'postcard' -- i get the valid tick

4) select more than one checkbox in the group and the valid tick appears

5) deselect all and the tick stays until you reselect one

Also, once you've force submitted the form to ajax (in any browser) with no checkboxes selected, the valid tick does not appear once any checkboxes are selected.