Sounds that you saved the compressed file as send by the server and that Firefox didn't decompress the file for some reason.

It is possible that (anti-virus) security software is causing the problem. Try to disable security software temporarily to see if that makes a difference.

i havent compressed it manually i follow normal protocal of downloading on firefox.can i decompress it. It didnt work out ,even when i download such files on android through firefox same thing happens is their any other solution.

Was the file you saved as .html originally a PHP webpage?

I am not sure the webpage from which i downloaded had aspx extension at the end ;the file has html extension. after reading your reply ,I saved it as php extension and then opened in the browser. it is showing as pdf, but with messed up layout not as originally saved. I am facing same problem in the android firefox too. i have attached the properties of file.