setPreference() property not being set?

Question

Java

Test environment:
Firefox ver.47.0.2
Selenium WebDriver ver.2.53.1
Eclipse ver.Mars.2 Release (4.5.2)
Question:
To begin with, the default user.js file is missing in the Firefox profile. Why is that? Created it. Next, I run a code that should overwrite the default property "general.useragent.override" from the prefs.js file (although there is no such property in this file, also "why so?").

package com.webdriver.chapter4;

import org.openqa.selenium.firefox.FirefoxProfile;
import org.openqa.selenium.firefox.FirefoxDriver;

public class SettingPreferences {

  public static void main(String[] args) {
    FirefoxProfile profile = new FirefoxProfile();
    
    profile.setPreference("general.useragent.override", "Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_0 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8A293 Safari/6531.22.7");
    
    FirefoxDriver driver = new FirefoxDriver(profile);
    driver.get("http://www.google.by");
  }
}

As a result, after running the code for execution, the "general.useragent.override" property should appear in the user.js file with the value specified in the code above - but it is not there, the file is empty! Although the displayed version of the search engine is mobile, which is true.
Has anyone experienced this?

Answer 1

Looks like the site is "zero" or close to being in terms of answers - a month has passed since the question was posted. It's a pity...