Annotate some easy wins in the firefox namespace

Author: RenderMichael

dotnet/src/webdriver/Firefox/FirefoxAndroidOptions.cs

@@ -18,17 +18,20 @@
 // </copyright>
 
 using OpenQA.Selenium.Internal;
+using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 
+#nullable enable
+
 namespace OpenQA.Selenium.Firefox
 {
     /// <summary>
     /// Generates the capabilities for automating Firefox applications on Android
     /// </summary>
     public class FirefoxAndroidOptions : AndroidOptions
     {
-        private List<string> androidIntentArguments = new List<string>();
+        private readonly List<string> androidIntentArguments = new List<string>();
 
         /// <summary>
         /// Initializes a new instance of the <see cref="FirefoxAndroidOptions"/> class.
@@ -41,10 +44,7 @@ public FirefoxAndroidOptions(string androidPackage) : base(androidPackage)
         /// <summary>
         /// Gets a read-only list of the intent arguments set for this set of options.
         /// </summary>
-        public ReadOnlyCollection<string> AndroidIntentArguments
-        {
-            get { return this.androidIntentArguments.AsReadOnly(); }
-        }
+        public ReadOnlyCollection<string> AndroidIntentArguments => this.androidIntentArguments.AsReadOnly();
 
         /// <summary>
         /// Argument to launch the intent with. The given intent arguments are appended to the "am start" command.
@@ -59,6 +59,7 @@ public void AddIntentArgument(string argument)
         /// Arguments to launch the intent with. The given intent arguments are appended to the "am start" command.
         /// </summary>
         /// <param name="arguments">The arguments to add.</param>
+        /// <exception cref="ArgumentNullException">If <paramref name="arguments"/> is <see langword="null"/>.</exception>
         public void AddIntentArguments(params string[] arguments)
         {
             this.androidIntentArguments.AddRange(arguments);

dotnet/src/webdriver/Firefox/FirefoxCommandContext.cs

@@ -17,6 +17,8 @@
 // under the License.
 // </copyright>
 
+#nullable enable
+
 namespace OpenQA.Selenium.Firefox
 {
     /// <summary>

dotnet/src/webdriver/Firefox/FirefoxDriverLogLevel.cs

@@ -17,6 +17,8 @@
 // under the License.
 // </copyright>
 
+#nullable enable
+
 namespace OpenQA.Selenium.Firefox
 {
     /// <summary>