Fixes to FirefoxOptions.java (SeleniumHQ#3327)

* Fixes to FirefoxOptions.java

- Remove unneeded check for both prefs and profile being present:
  According to mozilla/geckodriver#421
  having both is valid.
- Add getProfile method:
  Allows for use in intermediate nodes that need to modify the profile
  e.g. to add additional certs or extensions.

* Fix type error.

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

@@ -136,6 +136,10 @@ public FirefoxOptions setProfile(FirefoxProfile profile) {
     return this;
   }
 
+  public FirefoxProfile getProfile() {
+    return this.profile;
+  }
+
   // Confusing API. Keeping package visible only
   FirefoxOptions setProfileSafely(FirefoxProfile profile) {
     if (profile == null) {
@@ -183,11 +187,6 @@ public DesiredCapabilities addTo(DesiredCapabilities capabilities) {
 
     Object priorProfile = capabilities.getCapability(PROFILE);
     if (priorProfile != null) {
-      if (!booleanPrefs.isEmpty() || !intPrefs.isEmpty() || !stringPrefs.isEmpty()) {
-        throw new IllegalStateException(
-          "Unable to determine if preferences set on this option " +
-          "are the same as the profile in the capabilities");
-      }
       if (!priorProfile.equals(profile)) {
         throw new IllegalStateException(
           "Profile has been set on both the capabilities and these options, but they're " +