"You don't have permission to access..." errors are characteristic of a firewall that protects the server against misuse and this firewall thinks something is suspicious about the HTTP request. If you use a proxy or VPN then try to use a direct connection. You can try to clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences) to see if that fixes it. If that doesn't work then you can try "Forget About This Site" in the right-click context menu of a history item.