This is an official eBay add-on for Firefox. The add-on helps you keep an eye on your eBay activity wherever you are on the web when you install the eBay Sidebar for Firefox. It’s a free tool built with eBay users in mind that will help you get more out of your buying and selling on eBay.
Here is a little more about the add-on from the developers:
Be a better bidder on eBay! You’re always securely signed in so you can check out your eBay progress whenever you like. Customize your eBay alerts so you stay on top of your bids. Be told when an eBay auction style format listing you’re bidding on is about to end.
If you use eBay a lot, you need to grab this Firefox extension. You can pick up the eBay Sidebar from the Firefox Add-ons website.