2.0.10 Ajax Add To Cart Problem in Firefox

I just installed updates 2.0.9 and 2.0.10.

When I have AJAX add-to-cart enabled, clicking on an add to cart button prompts a download or open dialogue box – “You have chosen to open: index.php”. I’m using Firefox 3.5.5.

If I open or save this file, it contains a JSON response (attached).

Once you cancel this download, the items do show up successfully in the cart after a navigating to a new page or refreshing the current one.

I turned off AJAX, and there are no problems, but I’d like to have AJAX on.

Any Ideas?