Unless I am mistaken, folks who keep having persistent unwanted ads have been the victim of having something installed on their computer they didn't ask for and (most) probably don't want.
I am using FF on a MAC, with
Avast and here's the Warning I get (just) on
certain* webpages on Tractorbynet:
Infection Blocked!
When I get a warning like this, the webpage is blocked (i.e stopped from loading) thus I can't see/read what the content is.
For instance, I cannot get to either of these two
* webpages:
http://www.tractorbynet.com/forums/news-feedback/251506-malware-blocked.html
http://www.tractorbynet.com/forums/news-feedback/253563-whats-up-virus-warinigs-when-4.html
While the Tractorbynet
terms of use webpage makes no mention of you agreeing to allow them to download & install add-ons to your browser, it's apparent something is attempting to do so via certain pages on the website.
My guess is once the gzip file is successfully downloaded and opened, it then installs an add-on that is causing some of these ads to appear with tremendous veracity.
P.S.
Another excellent tool to detect and remove Malware from a MAC is
Adware Medic, it's FREE (i.e.
donationware), does a check in about a minute and is quite good at finding things installed within your other applications.