Web Development

Hi to all,
 
I am having live web application, which some times log out due to post back in firefox only.
At that time Session become null and Log Out.
 
In Other browsers working fine. I am using Firefox 69.0.2 (Latest) version.
 
Log Out happnes into Live Clients or in Production server only. In web developement working fine.
I am having Old Web Application around 12 years old web site (Product) and successfully using 150+ clients.
 
Some clients facing this issue. I have checked at my end. Issue still persist in some pages of web site.
Any one faced same issue?
 
Is there any thing that i can check that issue in otherway ?
 
Thank You 

public class ApiSampleUploadController : ApiController
   {


       [HttpPost]
       public string Test(HttpPostedFileBase file )
       {

           return "Success";
       }


   }

@using (Html.BeginForm("Submit"))
 {
 /*code*/
     //FileUpload
       <div>
        <input type="file" name="file" id="uploadFile" />
      </div>
/*code*/
 }

$('#uploadFile').on("change", function () {
    var formdata = new FormData($('form').get(0));

    CallUpload(formdata);
});


function CallUpload(file) {
    console.log(file);
    $.ajax({
        url: '/api/ApiSampleUpload/Test',
        type: 'POST',
        data:  'file=' + file ,
        cache: false,
        processData: false,
        contentType: 'application/json',
        success: function () {
            alert('Success');
        },
        error: function () {
            alert('Error occured');
        }
    });
}

private static async Task TestWebAPI(string url)
{
    using (var client = new HttpClient())
    {
        using (var httpResponse = await client.GetAsync(url))
        {
            try
            {
                httpResponse.EnsureSuccessStatusCode();     //<=======EXCEPTION HERE
                string responsContent = await httpResponse.Content.ReadAsStringAsync();
            }
            catch (Exception e)
            {
                _logger.Log(LogLevel.Error, e);
            }
        }
    }
}

I need to in the end figure out how to compose PHP code. I've begun with learning HTML as CSS and am interested in what extent other individuals spent learning HTML and CSS before moving onto PHP. Simply searching for some heading and any tips you folks can give.