Updating .NET FirefoxOptions to not force specifying binary location

In the Marionette/Geckodriver case, the service executable can locate the
Firefox executable on its own when looking in default installation
locations. There is no need for the language bindings to go spelunking
through the registry in that case any longer because the bindings do not
have to directly launch the Firefox executable.

Author: jimevans

dotnet/src/webdriver/Firefox/FirefoxOptions.cs

@@ -1,4 +1,4 @@
-// <copyright file="FirefoxOptions.cs" company="WebDriver Committers">
+// <copyright file="FirefoxOptions.cs" company="WebDriver Committers">
 // Licensed to the Software Freedom Conservancy (SFC) under one
 // or more contributor license agreements. See the NOTICE file
 // distributed with this work for additional information
@@ -355,12 +355,15 @@ private Dictionary<string, object> GenerateFirefoxOptionsDictionary()
             }
             else
             {
-                using (FirefoxBinary executablePathBinary = new FirefoxBinary())
+                if (!this.isMarionette)
                 {
-                    string executablePath = executablePathBinary.BinaryExecutable.ExecutablePath;
-                    if (!string.IsNullOrEmpty(executablePath))
+                    using (FirefoxBinary executablePathBinary = new FirefoxBinary())
                     {
-                        firefoxOptions[FirefoxBinaryCapability] = executablePath;
+                        string executablePath = executablePathBinary.BinaryExecutable.ExecutablePath;
+                        if (!string.IsNullOrEmpty(executablePath))
+                        {
+                            firefoxOptions[FirefoxBinaryCapability] = executablePath;
+                        }
                     }
                 }
             }