I am going to use Fiddler for a website protected by Forms Authentication. Despite the fact, I have already logged in in IE & Chrome but still the result is a redirection to the login page when I use Fiddler Composer to send the requests.
By the way I have already checked the "Automatically Authenticate" option but still no change.
Is the a universal solution or a best practice for this kind of situations?
Fiddler is a REST-based client. It's intended to be used against APIs. MVC, while it follows some principles of REST, diverges namely on the matter of state. In MVC, you have a concept of a session, and that is whether the state of being authenticated is stored. On the client side, it's represented by a cookie, usually. I've never really tried to access authenticated pages of an MVC site via Fiddler, so I can't say for sure, but if it does have an ability to pass a cookie along with the request, then that is how you can proceed. Just get the cookie from your browser after authenticating with the site and go from there.
If Fiddler does not possess the ability to send cookies (which is a distinct possibility), then really you're out of luck. The only alternative would be to use cookieless sessions in MVC, which is a hugely bad idea and officially deprecated. However, it would allow you to use Fiddler at that point, because the authentication token is passed via a URL query string.