По имени окна:
[DllImport("User32.dll")]
static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
[DllImport("User32.dll")]
static extern int SetForegroundWindow(IntPtr hWnd);
IntPtr ptrFF = FindWindow(null, "Mozilla Firefox");
SetForegroundWindow(ptrFF);
SendKeys.SendWait("{F1}");
По названию процесса:
Process proc = Process.GetProcessesByName("firefox")[0];
IntPtr ptrFF = proc.Handle;
SetForegroundWindow(ptrFF);
SendKeys.SendWait("{F1}");