Receiving CRITICA ERROR! from velonexa

I am receiving pop-up messages indicating CRITICAL ERROR! and "Click here to remove them!" followed by a URL ending with velonexa.com They appear as though they are from Mozilla, but we know better. I try to close the window and, of course, without clicking the url or the Dismiss button, but they keep reappearing. There are three stacked windows, constantly appearing as the pop-up windows are closed. Any thoughts or recommendations to eliminate? I've tried shutting down and rebooting. Pretty confident these are not from you. They are also always on top. I try to do a screen capture or print screen and i only capture what is behind them. I know this is not generated by Mozilla, but thought you should at least be aware of them.