[Osiris]

Price Drop is a Firefox add-on that gives the user the option to track prices for selected items at the Amazon.com online store and notify him when the price drops. Besides keeping track of prices drops at the Amazon store it also notifies you if the price dropped after purchasing the item which helps getting a refund for the item that you bought because of the lower price.
The extension adds a ‘Price Drop. Track this item’ row on every single product page at Amazon. Clicking the row adds that item to the list of items that are watched for price drops. The list of tracked products can be viewed at anytime by opening it in Tools > PriceDrop and it is possible to delete items in that list as well.

The user will be notified in a popup window if a price drop has been recognized. A link to the product page at Amazon is given in that popup as well. Unfortunately the old price is not mentioned in there, only the new price is mentioned.
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Price Drop Extension
via Lifehacker