In your search bar press the Amazon logo and select your new search engine. You can close then reopen Mozilla Firefox after to confirm it worked. Feel free to reply if you need further help.

Starting with Firefox 23 the keyword.URL pref is no longer supported and it is no longer possible to specify the search engine for the location bar that way.
The search engine that is used in the location bar is the search engine that is selected in the search Bar on the Navigation Toolbar.
You can disable automatic searching via the location bar by setting keyword.enabled to false and use a (one letter) keyword search to use a specific search engine instead to search via the location bar.

Another possibility is to disable keyword search (keyword.enable = false) and use a (one letter) keyword search for searching via the location bar.
You can set a keyword for an installed search engine via Manage Search Engines (click the search engines icon on the search bar).
That also allows to switch easily between search engines when searching via the location bar and you also do not have a problem with one word searches.

See also:

• Keyword Search: https://addons.mozilla.org/firefox/addon/keyword-search/
  

So glad someone beat you to the answer - your "answer", to me, is pure gibberish. Thanks for trying. All I needed to do is deselect Amazon, in the side address bar, and select Google. Although I did not have to do that for the past 6 years.