One thread for each tab

Question

I have made a web browser without using web browser class(Displaying html response on textbox).I m dynamically creating tab pages by programming on CREATE NEW TAB event.Now i want my browser to have a separate thread for each tab.I used asyn and await for the implementation of new thread for each tab but it gave me error "Cross-thread operation not valid: Control 'Form1' accessed from a thread other than the thread it was created on." Here is my code:

   async void navigateToPage(string s)
    {
        Uri uri = null;
        if (!Uri.TryCreate(s, UriKind.Absolute, out uri) || null == uri) 
        {
            //Invalid URL
            MessageBox.Show("Please Enter valid URL");}
        else
        {
           HttpWebRequest request = WebRequest.Create(s) as HttpWebRequest;
           HttpWebResponse response =  (HttpWebResponse)request.GetResponse(); 

            if (response.StatusCode == HttpStatusCode.OK)
            {
                // Contains protocol headers associated with a request or response
                WebHeaderCollection header = response.Headers;
                var encoding = ASCIIEncoding.ASCII;
                using (var reader = new System.IO.StreamReader(response.GetResponseStream(), encoding))
                {
                    string responseText = reader.ReadToEnd();
                    if (focussedTab == null)
                    {
                        textBox1.Text = responseText;

                        }
                    else
                        {
                         if (focussedTab.Controls.ContainsKey("outputBox"))
                            {
                                TextBox textBox = (TextBox)focussedTab.Controls["outputBox"];
                                textBox.Text = responseText;
                            }
                    }

                    using (tw = new StreamWriter(@"history.txt", true)) 

                    {  //writing history on a file
                        tw.WriteLine(s+"" +System.DateTime.Now);
                        tw.Close();
                    }
                    // adding url to the list for the back/fwd 
                    backStack.Push(s);
                    fwdStack.Clear();
                }
            }
            else if (response.StatusCode == HttpStatusCode.NotFound)
            {
                MessageBox.Show("Not Found");
            }
            else if (response.StatusCode == HttpStatusCode.Forbidden)
            {
                MessageBox.Show("Forbidden");
            }
            else if (response.StatusCode==HttpStatusCode.BadRequest)
                    {
                MessageBox.Show("Bad Request");
            }
              await DisplayNewTab(); 
              }

           Task DisplayNewTab()

    {
        return Task.Factory.StartNew(() =>
        {
            try
            {
                TabPage newTab = new TabPage();
                newTab.Name = "tab";
                newTab.Text = "New Tab";
                TextBox txtbx = new TextBox();
                txtbx.Name = "outputBox";
                txtbx.Multiline = true;
                txtbx.Size = new System.Drawing.Size(754, 259);
                tb1.Text = "https://";
                newTab.Controls.Add(txtbx);
                focussedTab = newTab;
                tabControl1.TabPages.Add(newTab);
            }
            catch (Exception excMsg)
            {
                MessageBox.Show(excMsg.Message.ToString(), "Error");
            }
            }); 
        }
    }
       }