Laptops Lenovo G50- 45: No smooth browsing

Laptop link:

1) Browser: Firefox (updated)
2) Problem: Lag while browsing (Add ons installed: Adblock, IDM, Youtube Downloader)
3) Already tried: Reinstalling firefox, windows
4) Using ccleaner to clean junk files and other errors.
6) Irrelevant startup items disabled
7) Current RAM: 12GB (4GB + 8GB)
8GB RAM link:

Please advice