You get the same webbrowser on all your tabs because this is exactly what your code is written to do.
You iterate from zero to your tabs count, and at each step insert in last position a new Tab with the same webbrowser than on every other tab.
You should really put a breakpoint at the beginning of your AddTabItem() method and run it in debug mode; you would instantly understand what is going on.
Finally, if you need to keep several webbrowsers, one for each tab, you need to declare a
collection of them; you could use an Array, a List, a Dictionary, whatever, but you need a collection.
Examples:
private List<WebBrowser> _webBrowsers = new List<WebBrowser>();
private Dictionary<int, WebBrowser> _webBrowsers = new Dictionary<int, WebBrowser>();
But anyway the priority here is to correct your AddTabItem() method. Debug it, execute it line by line, and check for the values of your variables at each step. The solution will come easily after that, and you will have learned an important part of a developer's life : debugging.