I had a different version of this until a few minutes ago when I updated to Firefox 57 and it stopped working. Searched add-ons, found this, installed in a minute and worked the first time I typed in amazon.com.
WORKS FINE. (to previous commenters) Doesn't have a toolbar button-doesn't need one, it simply changes any www.amazon link you type or click on to www.smile.amazon... before you get there. Activates itself. (in response to developer--the last version of a smile fix never had a button --it's not the sort of thing you'd want to turn off and on, I don't think, so why people don't notice it's working by just looking at the URL and seeing "smile" is in it is beyond me....)