I probably wouldn’t use an add-on like this myself … but I’m sure my wife would.
Coupons are a great way of saving money on the Internet. They obviously work best if you can get a coupon for an online shopping site that you want to make a purchase on (and not the other way round). The Firefox extension CouponsHelper is an unobtrusive add-on that informs you about coupon codes on supported merchant websites. The extension displays a small notification at the top of the browser window that lists coupons for that merchant.