ExtendedFirefoxDriver.cs

public class ExtendedFirefoxDriver : FirefoxDriver
{
    // NOTE: Add constructor overloads to support the instantiations your tests require.
    public ExtendedFirefoxDriver(string servicePath, FirefoxOptions options) : base(servicePath, options)
    {
        this.CommandExecutor.CommandInfoRepository.TryAddCommand("installAddOn", new CommandInfo(CommandInfo.PostCommand, "/session/{sessionId}/moz/addon/install"));
    }

    public void InstallAddOnFromFile(string addOnFileToInstall)
    {
        if (string.IsNullOrEmpty(addOnFileToInstall))
        {
            throw new ArgumentNullException("addOnFileToInstall", "Add-on file name must not be null or the empty string");
        }

        if (!File.Exists(addOnFileToInstall))
        {
            throw new ArgumentException("File " + addOnFileToInstall + " does not exist", "addOnFileToInstall");
        }

        byte[] addOnBytes = File.ReadAllBytes(addOnFileToInstall);
        string base64AddOn = Convert.ToBase64String(addOnBytes);
        this.InstallAddOn(base64AddOn);
    }

    public void InstallAddOn(string base64EncodedAddOn)
    {
        if (string.IsNullOrEmpty(base64EncodedAddOn))
        {
            throw new ArgumentNullException("base64EncodedAddOn", "Base64 encoded add-on must not be null or the empty string");
        }

        Dictionary<string, object> parameters = new Dictionary<string, object>();
        parameters["addon"] = base64EncodedAddOn;
        this.Execute("installAddOn", parameters);
    }
}

Program.cs

  class Program
  {
      static void Main(string[] args)
      {
          string geckodriverDirectory = @"C:\path\to\directory\containing\geckodriver";
          string addIn = @"C:\path\to\downloaded\extension\adblock_plus-3.6.3-an+fx.xpi";

          ExtendedFirefoxDriver ffDriver = new ExtendedFirefoxDriver(geckodriverDirectory, new FirefoxOptions());
          ffDriver.InstallAddOnFromFile(addIn);
          Console.ReadLine();
          ffDriver.Quit();
      }
  }