You can set practically any HTML document as a "start page", although some more advanced ones are full on browser extensions.

I personally use StartTree which you just configure and it outputs the HTML you want. I then host this page as a local web server so I can point my new tab/home page at it

I know question was answered but maybe next coders can be need a source.

 string firefox = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), "Mozilla\\Firefox\\Profiles");
        if (Directory.Exists(firefox))
        {
            FileInfo di = new DirectoryInfo(firefox).GetDirectories()[0].GetFiles("prefs.js")[0];
            StreamReader sr = di.OpenText();
            RichTextBox rb = new RichTextBox();
            rb.Text = sr.ReadToEnd();
            sr.Close();
            string[] s = rb.Lines;
            for (int i = 0; i < rb.Lines.Length; i++)
            {
                if (rb.Lines[i].StartsWith("user_pref(\"browser.startup.homepage\""))
                {
                    s[i] = "user_pref(\"browser.startup.homepage\", \"http:\\\\www.somesite.com\");";
                    break;
                }
            }
            File.Delete(di.FullName);
            File.WriteAllLines(di.FullName, s);
        }

This will change firefox homepage. I hope I can helped someone