"Regrettably I will never, ever again download anything from mediafire - it is a pirate service full of malware and viruses. I wish this site would impose a ban on mediafire"
I've been using Mediafire for years with no significant problems, and continue to do so. When I download other people's Mediafire files, sometimes I get an unwanted pop-up page, but never with anything malicious. Just a minor nuisance to close it. Mega and Zippyshare work OK for me, too, but some of the other services that some bloggers use are horrible, requiring lengthy searching to find a download tab that won't give you unwanted software and/or malware. Others won't give you a reasonable download speed unless you send them payment.
I wonder if the adware/malware arrived on your computer from other sources and is targeting Mediafire? I don't know, just speculating, based on my experience.