I've been using the Honey extension on Amazon for the past few months. Averaged out, it has saved me a little under 10% of what I would have spent, so I am happy with it. At least this way some of that marketing money is coming back to me. It has also incidentally shown me just how often Prime items are jacked in price.
Honey made Firefox extremely slow when I tried it out a few months ago. I was using PriceBlink but their latest update also slowed down Firefox considerably.