Shopping cart widget misbehaving in Firefox.

Hello,
I am using the latest version of both Marketpress and Firefox. For some reason the shopping cart widget is misbehaving using this browser: 1) the shopping cart does not update when a product is added and 2) when clicking on "empty cart" the widget displays a replica of my homepage.
How can this be solved?
Thank you for your time!
Warmly,
Nancy