How do I change user interface language in Firefox to avoid so-called localization?

You should install an English language pack. A language pack is a Firefox add-on.

You can download it from here. Then you must install it like any other add-on ("install add-on from file" in the add-on interface).

Then you should change the language preference. You can do this by typing about:config in the Location bar (address bar) and then searching for

general.useragent.locale

And change that to:

en-US

(the same name as the downloaded .xpi) Then you must restart the browser.

More details at: http://kb.mozillazine.org/Language_packs

Install en-US https://addons.mozilla.org/en-US/firefox/addon/english-us-language-pack/ and have key intl.locale.requested set to en-US in about:config.

https://support.mozilla.org/en-US/kb/use-firefox-interface-other-languages-language-pack