Change browser.search.region to US in about:config then restart the browser.

I don't want us amazon, but uk amazon.

Anyway where is about:config ?

Only Amazon.com is available as one of the built-in search engines (browser.search.region => US), other locales have been removed for some reason. For other locales you can visit the Mycroft Project website.

• https://mycroftproject.com/search-engines.html
• https://mycroftproject.com/amazon-search-plugins.html

You can open the about:config page via the location/address bar. On the warning page, click "Accept the Risk and Continue" to open about:config.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox