There are a few sites that disallow browsing unless you are using a specific browser, such as Internet Explorer. The User Agent Add-on will get around that in many cases. When you arrive at a site and see a message that says, in effect, that they only serve Internet Explorer, or some other browser, just bring down User Agent’s menu and select the appropriate browser. Re-load that site with F5.
You can get User Agent
here. 
firefox
