How to find out which process is using localhost:80?

I'm using linux mint xfce edition, my localhost:80 was used by some program but I don't which one, when I open firefox and visit localhost:80, it says

It works! This is the default web page for this server. The web server software is running but no content has been added, yet.

I've tried to use lsof -i @localhost:80, but it returns nothing.