How to deal with possible malware problems

Hi all, both my wife's machine and mine are running the same ver of Norton Internet Security product and keep it updated and scanned regularly.

I have not noticed any problems on my machine up to this point.

Her machine is however.

When she pulls up 'firefox' and checks her mail in one tab, and looks at 'facebook timeline' in another tab, she gets a screen that will pop-up, fill the whole screen, and say stuff about 'warning call this number because a malware/adware virus has been activated on your computer (877/695-1763)

The web address that autofills in for this pop-up screen is from: malwaretechsupport.org

When this screen pops-up, a voice comes on and tells you all this. To call immediately to get the problem fixed.

Since I have been through diff malware things in the past, i'm wondering if there is a base set of malware tools that I should have loaded on both our machines which i can run semi-regularly to see if things are ok or not.  Also, am I correct in understanding that Norton Internet Security is basically for viruses, etc. and not really for the prevention of malware and other types of stuff?

I have here trying to see if this is repeatable on demand or not still.  But it has just started happening today, and have had no issues with her machine in the past.

We are both running Win 8.1 and have had not problems with anything up to this point.

Thanks for your assistance.