I encountered this issue when trying to sign in using Firefox and although hybridauth was working great on all browsers, it was failing on Firefox browsers. so if you are looking for an answer to 
hybridauth firefox You cannot access this page directly
 here is the solution.
the problem is that Firefox has a caching mechanism for page redirects, you can find out more about this topic on this thread firefox-5-caching-301-redirects
as a result, if you are trying to authenticate a user on a login page and the cache headers for that page were in the range of 30x (301-302-303-304...) firefox is more likely to cache that page on the client side.
So when a user tries to sign in, thelogin page will not be processed on your server, instead firefox will serve a cached version of the login page which redirects to the hybridauth authentication page, failing in turn to initialize the Session Object required for authentication.
Solution: turn off the caching headers for the login page and make sure that it returns a 200 code