I have a very frustrating issue that is driving me round the bend!

I have a contact form that can be seen here. At the bottom of the form, to the left of where it says *Champs obligatoire, there is a an outlined box which is meant to be the submit button!

The button has a class of 'submit-btn' on it but for some reason it is picking up another style of 'outline: 1px solid #ff0000' (which I can see using one of the FF add-ons) but I have no idea where this is coming from!

I've done a search of all my external style sheets and there is nothing with a style value of outline. There is also no inline styling around the button!

Has anyone come across anything like this before? I'm at a loss as to where to look next!