The implementationClass property is ignored when using WebDriver interface as type of selenium instance in tests

I have this configuration for webdriver in arquillian.xml:

<extension qualifier="webdriver">
   <property name="implementationClass">org.openqa.selenium.firefox.FirefoxDriver</property>
</extension>

and this is declaration of my selenium instance:

@Drone
protected WebDriver webDriver;

Since I am not using special properties in arquillian.xml for firefox driver (such as firefoxBinary or firefoxProfile), it should be enough to declare selenium instance with type WebDriver interface.

However, in this configuration, it is ignoring the implementationClass property, and instead of FirefoxDriver it runs default HTMLUnitDriver.

It runs Firefox when the selenium instance is declared with the type FirefoxDriver.