prosites paypal unsecure firefox

Hi, when I use firefox and the paypal express checkout I am getting an un-secure connection message (see attached screenshot). These are my details:

I have an SSL installed on my main site (not wildcard)
I am using the PayPal live version
I have set my IPN to my https address in PayPal
PayPal header image is delivered from a https connection
I do not get this warning in IE or Chrome

I really need to sort this out as the message is very off putting. If I continue then all is fine but my customers aren't going to do this. Any ideas??

Thanks
Graham J OConnor