Safari Hijacked and Certificates that won't go away. Malware?

Question

Four times over the last 4 or 5 months "something" hijacks Safari and freezes it.
The first 3 times I was in Google.   I clicked on a site, it would freeze Safari and I would get a message from an unrelated website telling me to call a number for Apple Support which was not Apple.
Each of those 3 times I called Apple Care and they went to Finder>Library> Cache and removed files from Launch Agents, Launch Daemons and Plug-ins.  That removed the problem.
This time I was in Words With Friends when a Certificate popped up for the website Secure.RubiconProject.Com.   I spent over an hour on the phone with Apple looking for bad files.  None were found but we couldn't get rid of the Certificate and Safari was frozen.   Also, WWF would keep crashing Safari.
The Apple guy had me install a 2 week free trial of Web Root antivirus.  It found some problems which it took care of.  When I went back to Words with Friends the Certificate popped up again but this time I clicked on it since Web Root would tell me if there was an issue. 
The screen was blank but since then: 1.  No certificate and 2. an error message that was always across the top of WWF about having to reload the page was gone.  When I used Firefox to work on this problem with Apple, I would keep getting a message that said Firefox would not direct this to another page and I had to click an OK button.  That is gone.
I don't know why I keep getting this issue since each time it is different.   I told Apple guy that I had read that Apple doesn't need antivirus and in fact they can cause more problems than they help.   He said Apple has a Firewall but nothing for malware which is become more and more prevalent.  There are threads here about not needed anti viruses but I'm at a loss. 
I have Apple Care for 2 more years but I don't want to keep calling them if I can find a solution to this.  I'm not tech savvy.   I would like an antivirus that is a one time fee. 
Advice please?

Answer 1

You may have inadvertently installed adware. Eradicating it is simple and you don't have to download or install anything to fix it.
Although adware relies upon deception, it does not get installed without your consent, and "anti-virus" utilities cannot prevent users from willfully installing garbage. Only you can do that, by recognizing adware's appearance, which is constantly changing as adware authors constantly attempt to thwart automatic means of detection.
For an explanation or how this may have occurred, how to avoid it in the future, and for Apple's recommended solution read How to install adware.
Webroot is garbage that won't help prevent adware. Uninstall it. Be sure to follow its uninstallation instructions.
I have Apple Care for 2 more years but I don't want to keep calling them if I can find a solution to this.
You should call them, as often as necessary. AppleCare is a service you paid for. Let them work for you. I suggest you express your displeasure regarding their inept recommendation to install Webroot. OS X's software firewall is also completely irrelevant to your concern.