class Program {
       [DllImport("user32.dll", SetLastError = true)]
       private static extern bool MoveWindow(IntPtr hWnd, int X, int Y, int nWidth, int nHeight, bool bRepaint);

       [DllImport("user32.dll", SetLastError = true)]
       private static extern bool EnumWindows(EnumWindowsProc lpEnumFunc, IntPtr lParam);

       [DllImport("user32.dll", SetLastError = true)]
       private static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);

       [DllImport("user32.dll")]
       [return: MarshalAs(UnmanagedType.Bool)]
       private static extern bool IsWindowVisible(IntPtr hWnd);

       [DllImport("user32.dll", SetLastError = true)]
       private static extern IntPtr GetWindow(IntPtr hWnd, uint uCmd);

       [DllImport("user32.dll", SetLastError = true)]
       private static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);

       [DllImport("user32.dll", SetLastError = true)]
       private static extern int GetClassName(IntPtr hWnd, StringBuilder lpClassName, int nMaxCount);

       private const uint GW_OWNER = 4;
       private const int MaxRetries = 100;
       private const int SleepTime = 200;
       private const string ClassNameToFind = "MozillaWindowClass";

       private delegate bool EnumWindowsProc(IntPtr hWnd, IntPtr lParam);

       private static IntPtr foundWindowHandle = IntPtr.Zero;

       private static bool EnumWindowsCallback(IntPtr hWnd, IntPtr lParam) {
           uint processId;
           GetWindowThreadProcessId(hWnd, out processId);

           if (IsWindowVisible(hWnd) && GetWindow(hWnd, GW_OWNER) == IntPtr.Zero) {
               StringBuilder className = new StringBuilder(256);
               GetClassName(hWnd, className, className.Capacity);

               if (className.ToString() == ClassNameToFind) {
                   foundWindowHandle = hWnd;
                   Console.WriteLine($"Found window handle {hWnd} for process ID {processId}");
                   return false; // Stop enumerating windows once the desired window is found
               }
           }

           return true; // Continue enumerating windows
       }

       static void Main(string[] args) {
           string path = @"C:\Program Files\Mozilla Firefox\firefox.exe";

           // Define positions and sizes
           int[,] positions = new int[,]
           {
               {0, 0, 1284, 2120},
               {1280, 0, 1284, 2120},
               {2560, 0, 1284, 2120}
           };

           int initialDelay = 750; // Initial delay in milliseconds, 1000 worked
           int retryDelay = 200; // Delay between retries in milliseconds

           // Start three Firefox processes sequentially and move them to specified positions
           for (int i = 0; i < 3; i++) {
               Process firefoxProcess = Process.Start(path);
               Thread.Sleep(initialDelay); // Wait for the process to initialize

               foundWindowHandle = IntPtr.Zero;
               int retries = 0;

               while (foundWindowHandle == IntPtr.Zero && retries < MaxRetries) {
                   EnumWindows(EnumWindowsCallback, IntPtr.Zero);
                   if (foundWindowHandle == IntPtr.Zero) {
                       Thread.Sleep(retryDelay); // Wait before retrying
                       retries++;
                   }
               }

               if (foundWindowHandle != IntPtr.Zero) {
                   bool success = MoveWindow(foundWindowHandle, positions[i, 0], positions[i, 1], positions[i, 2], positions[i, 3], true);
                   if (success) {
                       Console.WriteLine($"Moved window {i + 1} to X: {positions[i, 0]}, Y: {positions[i, 1]}, Width: {positions[i, 2]}, Height: {positions[i, 3]}");
                   } else {
                       Console.WriteLine($"Failed to move window {i + 1}");
                   }
               } else {
                   Console.WriteLine($"Failed to get handle for Firefox process {i + 1}");
               }
           }
       }
   }

modify to suit your tastes or improve it? working now without any known issues. using System.Diagnostics; using System.Runtime.InteropServices; using System.Text; namespace FirefoxWindowPositioner { class Program { [DllImport("user32.dll", SetLastError = true)] private static extern bool MoveWindow(IntPtr hWnd, int X, int Y, int nWidth, int nHeight, bool bRepaint); [DllImport("user32.dll", SetLastError = true)] private static extern bool EnumWindows(EnumWindowsProc lpEnumFunc, IntPtr lParam); [DllImport("user32.dll", SetLastError = true)] private static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId); [DllImport("user32.dll")] [return: MarshalAs(UnmanagedType.Bool)] private static extern bool IsWindowVisible(IntPtr hWnd); [DllImport("user32.dll", SetLastError = true)] private static extern IntPtr GetWindow(IntPtr hWnd, uint uCmd); [DllImport("user32.dll", SetLastError = true)] private static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount); [DllImport("user32.dll", SetLastError = true)] private static extern int GetClassName(IntPtr hWnd, StringBuilder lpClassName, int nMaxCount); private const uint GW_OWNER = 4; private const int MaxRetries = 100; private const int SleepTime = 200; private const string ClassNameToFind = "MozillaWindowClass"; private delegate bool EnumWindowsProc(IntPtr hWnd, IntPtr lParam); private static IntPtr foundWindowHandle = IntPtr.Zero; private static bool EnumWindowsCallback(IntPtr hWnd, IntPtr lParam) { uint processId; GetWindowThreadProcessId(hWnd, out processId); if (IsWindowVisible(hWnd) && GetWindow(hWnd, GW_OWNER) == IntPtr.Zero) { StringBuilder className = new StringBuilder(256); GetClassName(hWnd, className, className.Capacity); if (className.ToString() == ClassNameToFind) { foundWindowHandle = hWnd; Console.WriteLine($"Found window handle {hWnd} for process ID {processId}"); return false; // Stop enumerating windows once the desired window is found } } return true; // Continue enumerating windows } static void Main(string[] args) { string path = @"C:\Program Files\Mozilla Firefox\firefox.exe"; // Define positions and sizes int[,] positions = new int[,] { {0, 0, 1284, 2120}, {1280, 0, 1284, 2120}, {2560, 0, 1284, 2120} }; int initialDelay = 750; // Initial delay in milliseconds, 1000 worked int retryDelay = 200; // Delay between retries in milliseconds // Start three Firefox processes sequentially and move them to specified positions for (int i = 0; i < 3; i++) { Process firefoxProcess = Process.Start(path); Thread.Sleep(initialDelay); // Wait for the process to initialize foundWindowHandle = IntPtr.Zero; int retries = 0; while (foundWindowHandle == IntPtr.Zero && retries < MaxRetries) { EnumWindows(EnumWindowsCallback, IntPtr.Zero); if (foundWindowHandle == IntPtr.Zero) { Thread.Sleep(retryDelay); // Wait before retrying retries++; } } if (foundWindowHandle != IntPtr.Zero) { bool success = MoveWindow(foundWindowHandle, positions[i, 0], positions[i, 1], positions[i, 2], positions[i, 3], true); if (success) { Console.WriteLine($"Moved window {i + 1} to X: {positions[i, 0]}, Y: {positions[i, 1]}, Width: {positions[i, 2]}, Height: {positions[i, 3]}"); } else { Console.WriteLine($"Failed to move window {i + 1}"); } } else { Console.WriteLine($"Failed to get handle for Firefox process {i + 1}"); } } } } }