problems with a virus message

I've just had a virus message pop up in Firefox, with the following URL in the URL bar:

The on-screen message was:

Needless to say I did not click ok - it may, or may not, be a trap.

Any thoughts on this?

I don't think I linked to any dodgy sites, but how could I be sure?

Using OSX 10.10, and Firefox 39.0

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