Authentication issue with Firefox

I am having an issue with my authentication with firefox.  Everything works fine in IE.  I am logged into my website as "User1" and when I click on the logout control, the page just does a postback and nothing happens.  I am still logged in as "User1".  My logout control is in a user control which is in a master page. 
When I go to my login screen and try logging in as "User2", it acts as though it works and redirects me correctly, but I am still logged in as "User1".
Has anyone experienced this same issue?  I have included my logout code.  Please let me know if you need to see more code.
Protected Sub lnkLogOut_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles lnkLogOut.Click Dim alUsers As System.Collections.Generic.List(Of String) If Parent.Page.User.Identity.IsAuthenticated Then alUsers = ...