Default firefox in tests to use marionette

It's about time to do this.

Author: shs96c

java/client/test/org/openqa/selenium/testing/drivers/BrowserToCapabilities.java

@@ -37,8 +37,10 @@ public static DesiredCapabilities of(Browser browser) {
 
       case ff:
         caps = DesiredCapabilities.firefox();
-        caps.setCapability(FirefoxDriver.MARIONETTE,
-                           Boolean.getBoolean(FirefoxDriver.SystemProperty.DRIVER_USE_MARIONETTE));
+        String property =
+          System.getProperty(FirefoxDriver.SystemProperty.DRIVER_USE_MARIONETTE, "true");
+        boolean useMarionette = property != null && Boolean.parseBoolean(property);
+        caps.setCapability(FirefoxDriver.MARIONETTE, useMarionette);
         break;
 
       case htmlunit:

java/client/test/org/openqa/selenium/testing/drivers/SynthesizedFirefoxDriver.java

@@ -67,10 +67,6 @@ private static Capabilities tweakCapabilities(Capabilities desiredCaps) {
     }
     DesiredCapabilities tweaked = new DesiredCapabilities(desiredCaps);
 
-    if (!Boolean.TRUE.equals(tweaked.getCapability(MARIONETTE))) {
-      tweaked.setCapability(MARIONETTE, false);
-    }
-
     if (tweaked.getCapability(PROFILE) == null) {
       tweaked.setCapability(PROFILE, createTemporaryProfile());
     } else {