You most likely visited a malware domain which ran an animation purporting to show your machine is infected with viruses. Here's an example of one currently making the rounds: http://www.bleepingcomputer.com/virus-removal/remove-win-7-antispyware-2011 Malwarebytes will have deleted the rogue app. I'd advise you to install this add-on: https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/ Subscribe to the "Easylist" option when you see it during the installation, then go to https://adblockplus.org/en/subscriptions and scroll down to the foot of the list. You'll find a link there where you can subscribed to "'''Malware Domains'''" to prevent a recurrence.

Many thanks, done that immediately

You're welcome.