how to get ID process when it is started a new instance?

I am trying to start Firefox and after few seconds, close it. My code is this:

using (Process myProcess = new Process())
{
    myProcess.StartInfo.UseShellExecute = false;
    myProcess.StartInfo.FileName = @"C:\Program Files\Mozilla Firefox\firefox.exe";
    myProcess.StartInfo.CreateNoWindow = false;
    myProcess.Start();

    Task.Delay(5000);


    Process.Start("taskkill.exe", $"/PID {myProcess.Id}");
}

Then problem is that when i set a break point in the taskkill process, myProcess.Id doesn't exist in the task manager. All the instances of Firefox has another ID.

If I try to open Notepad, it works, the ID of Notepad is the ID of myProcess.

So I am thinking that perhaps with Firefox it is opened a process, it is closed and then another instance of Firefox it is opened. Is this true?

How could know the ID fo firefox? Or how could I kill all the process of Firefox? Because in fact I don't have only one Firefox process, I have many.

Thanks.

When using Task.Delay(...) you should await for it otherwise you start a new task and forget it without any delay:

Your code amended:

//TODO: check if your method is declared as async
private async Task MyMethod() {

  ...

  using (Process myProcess = new Process()) {
    myProcess.StartInfo.UseShellExecute = false;
    myProcess.StartInfo.FileName = @"C:\Program Files\Mozilla Firefox\firefox.exe";
    myProcess.StartInfo.CreateNoWindow = false;
    myProcess.Start();

    // await for 5 seconds delay
    await Task.Delay(5000);

    // this will be continued after 5 seconds delay 
    // Do not forget to dispose yet another process - taskkill.exe
    // Why not myProcess.Kill(); ?
    using (Process.Start("taskkill.exe", $"/PID {myProcess.Id}")) {}
  }

Code: (my version)

//TODO: check if your method is declared as async
private async Task MyMethod() {
  ...
  
  // Let's extract model (how to start process) from execution
  ProcessStartInfo psi = new ProcessStartInfo() {
    UseShellExecute = false,
    FileName = @"C:\Program Files\Mozilla Firefox\firefox.exe",
    CreateNoWindow = false,
  };

  // Process.Start can return null, which we forgive with !
  using (Process myProcess = Process.Start(psi)!) {
    // We, typically, cancel Tasks instead of delaying and analyzing 
    using (CancellationTokenSource cts = new CancellationTokenSource(5000)) {
      try {
        // start process with cancellation after 5 seconds 
        await myProcess!.WaitForExitAsync(cts.Token);
      }
      catch (TaskCanceledException) {
        // if process has been cancelled (not completed) we want to kill it
        myProcess!.Kill();
      }
    }
  }