When under a sold listing in eBay where the seller has accepted a Best Offer price, clicking the WDISF? button in Firefox will display a small window in your browser showing the price the seller accepted for their listing, it will also show the form of currency paid.
In order for this add-on to work you must be under the "Sold" category on eBay when searching an item. Look for an item that was sold using the best offer option (the price will have a slash mark through it). Make sure to click "See original listing".