The problem occurs on 
each page example: containing JavaScript(?)/Flash(?) (a very long time does not show images before will not load)

 <=== image load issue
For comparison, Chrome loads instantly.
We do not know how to solve this problem and it practically after every update FF - the same problem.
I want to use FF, 
but loads pictures, several hundred times slower than Chrome. 
EXAMPLE YOUTUBE:Issue: There are no video thumbnail 
Edit: There is a high probability that the change helps the user agent (eg. In this case) 
Windows / Chrome 47: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/47.0.2526.106 Safari/537.36 Replace to Windows / IE 11: Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Geckohttp://s14.postimg.org/gww077o0h/Screen_Shot_01_04_16_at_11_18_AM.jpg