Rated 3 out of 5 stars

I prefer greasemonkey script for Firefox called BookInfoLine.

It always works with any FF update.
Way more sites are searched. It is faster. In any case here is what they say:

Script Summary:
Skip extra time and steps of going to "price comparison sites". Compare price of the book you are looking at on your favourite site in seconds against various book stores: Abebooks, Alibris, Amazon.com, Barnesandnoble, Betterworldbooks, Biblio, Borders, Craigslist, Ebay, Ebooks, Google Books, Half, Kindle Editions, Powells, Strandbooks, Thriftbooks, Walmart. Fixed and Improved BookBurro. It will hover as yellow rectangle named BookInfoLine all you will need to to is click small arrow and it will unfold and show you all the price, when done click the arrow again and it will fold back into unobtrussive small rectangle.

This review is for a previous version of the add-on (0.74).