Virus / malware and how to fix it

It is definitely not a virus, there are no (known) viruses for OS X.

What about a different browser, Apple's own Safari is fast, overall it works well.

 

It's only on Firefox and works fine on Safari? That helps narrow it down. I would disable any extensions, add-ons, browser toolbars, anything you may have changed from stock Firefox. If that works, narrow down to which of those disabled items was causing it and remove.

 

I'm not on a machine running Firefox so I can't walk through the menus, but it should be under preferences or similar. You may need to look at plugins, extensions, or anything else that could be an add-on to the browser.

 

If you haven't already done so, try changing your DNS servers on your Mac and your router to OpenDNS servers. This will show you how: Why am I being redirected to other sites?

Macs are not immune to malware, but no true viruses exist in the wild that can run on Mac OS X, and there never have been any since it was released over 10 years ago. The only malware in the wild that can affect Mac OS X is a handful of trojans, which can be easily avoided by practicing safe computing (see below). 3rd party antivirus apps are not necessary to keep a Mac malware-free, as long as a user practices safe computing, as described in the following link. Also, it is not recommended that you install more than one antivirus app, as they can conflict with each other and cause false positives. Read the What security steps should I take? section of the Mac Virus/Malware FAQ for tips on practicing safe computing.