Firefox should be configured to notify you of coupons when you shop on sites like Amazon. I think that would be a great idea. Being able to go into any online store and see that the store has coupons would be great!
...a Firefox Product Manager just kicked off a discussion here on Connect about shopping online/improving the shopping experience on Firefox. And you're all invited to share join and share your thoughts. Here is a link to the thread: Share your thoughts on how you shop online!