Do you have any extensions on Firefox that could be causing this? Either way, send a message to staff via Special:Contact, this should be looked into.
I have a bunch of extensions, but only those approved by Firefox. The site has been loading slow tonight, too. Nothing new, but it raised a flag when I got the virus notice. Possibly related: Recent Changes hasn't been loading on the iPad at all in the past day or so.
Wikia doesn't get viruses. It may be an extension on your browser, or a virus that has been infecting your computer. Yes, Wikia is slow sometimes but it doesn't give our viruses.
Whilst it's true that "any computer can get a virus" - if Wikia is sensible, then they probably aren't going to "get a virus" unless there's a really major bug in probably more than one system at the same time.
What's more likely is one of 3 things:
Normally, your system is ok, and unless you've changed anything, if you're running a modern Operating System (Windows Vista/7/8, Mac or Linux) - there's a lot of security that stops malicious programmes installing themselves. Being vigilant and not opening attachments in emails, or running programmes you don't trust will reduce your chance of getting a virus down to a very low level.
I'd recommend you reboot your computer and run a full AV check in safe mode to be sure though.