Adtech Malware windows in Safari that won't close HELP!

I always have used Firefox. Recently I decided to use Safari as since I updated to Yosemite on my Imac and it's been slow since, it's a (Mid 2011 4gb Ram 2.7ghz I5 500 in Memory but only 1/3 used). I also use Pro Tools 11. Yosemite has caused some new issues with that running too. More CPU errors. I likely have to buy some RAM.

Having said that I've never had anything like this happen in Firefox. Currently Safari is frozen with this Adtech malware ad asking to call a 1800 #. If you hit OK it goes to more and more windows trying to get you to call this # but you can't operate any other part of Safari. You can only Quit.

Firefox runs just fine. I've deleted the Cache and all cookies but I'm in Fire fox. I want this Adtech thing gone. How do I get rid of that When I open Safari???

Thanks