FirefoxResponsiveMode.java

package web.acceptance.utility;

import org.openqa.selenium.Keys;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.firefox.FirefoxProfile;
import org.openqa.selenium.interactions.Actions;

public class FirefoxResponsiveMode {

    public static void pressShortcut(WebDriver driver) {
        Actions actions = new Actions(driver);
        actions.keyDown(Keys.CONTROL).keyDown(Keys.SHIFT).sendKeys("m").keyUp(Keys.SHIFT).keyUp(Keys.CONTROL).perform();
    }

    public static void setViewSize(FirefoxProfile profile, String viewDeviceName, Integer width, Integer height) {
        String responsiveMode =
            "[{\"width\": " + width + ", \"name\": \"" + viewDeviceName + "\", \"key\": \"" + width + "x" + height + "\", \"height\": "
                + height + "}]";
        profile.setPreference("devtools.responsiveUI.presets", responsiveMode);
    }
}

I used your code . Firefox opens in the mobile mode , but in default size and not the size specified in setViewSize