Go here: to Book Burro and install the add-on. Restart your FF browser. Now when you search for a book, say Orson Scott Card's Ender's Game at a website, Book Burro will pop up a small, see through bar. If you click on the bar, this is what you will see.
Note it shows you local libraries in your area that have this book, as well as the price of several online booksellers for you to compare with and find the best price.
Important Note: Book Burro doesn't pop up until you actually click onto an individual book's page. If you are on a search page (like on Amazon.com or BN.com) showing multiple books, it will not show up until you select a single book.