System Property DRIVER_USE_MARIONETTE if set in java or command line …

…arg with server will ignore desired capabilities and force marionette when true or force legacy firefox driver when false. Default unset, listens to capabilities requested.

java/client/src/org/openqa/selenium/firefox/FirefoxDriver.java

@@ -97,7 +97,8 @@ public static final class SystemProperty {
     public static final String DRIVER_XPI_PROPERTY = "webdriver.firefox.driver";
 
     /**
-     * Boolean system property that instructs FirefoxDriver to use Marionette backend.
+     * Boolean system property that instructs FirefoxDriver to use Marionette backend,
+     * overrides any capabilities specified by the user
      */
     public static final String DRIVER_USE_MARIONETTE = "webdriver.firefox.marionette";
   }
@@ -284,10 +285,22 @@ public Timeouts setScriptTimeout(long time, TimeUnit unit) {
   }
 
   private static boolean isLegacy(Capabilities desiredCapabilities) {
+    Boolean forceMarionette = forceMarionetteFromSystemProperty();
+    if (forceMarionette != null) {
+      return !forceMarionette;
+    }
     Object marionette = desiredCapabilities.getCapability(MARIONETTE);
     return marionette instanceof Boolean && ! (Boolean) marionette;
   }
 
+  private static Boolean forceMarionetteFromSystemProperty() {
+    String useMarionette = System.getProperty(SystemProperty.DRIVER_USE_MARIONETTE);
+    if (useMarionette == null) {
+      return null;
+    }
+    return Boolean.valueOf(useMarionette);
+  }
+
   @Override
   protected void startClient(Capabilities desiredCapabilities, Capabilities requiredCapabilities) {
     if (isLegacy(desiredCapabilities)) {