I've started getting pop-under ads every time I launch Firefox, and the uninstall instructions don't make sense, how can I stop these?

In the past couple of weeks, every time I launch Firefox, a full-size pop-under ad is also launched, regardless of what site I'm browsing.

The original ad was for a You Tube survey, and contains a link to, where there's a menu option for uninstalling. The uninstall instructions don't make sense, they don't mention what ad-on is controlling the ads, and I can't work out what's responsible for the ads from my very basic range of ad-ons.

The reason I'm writing is that the ads have progressed from annoying but harmless, to dangerous. The ad today was designed as a warning pop-up, text reads, "Threat Detected: Immediate Action Required. Fatal Registry Error ... click "Repair All" to eliminate all possible threats and clean your PC". I have screenshots of all the above.

I'm worried that another user, my son or mother, might take this warning seriously and install malware by accident. What can I do to prevent these ads? I expect this type of malware advertising on dodgy websites, but I'm getting them when I try to access hotmail, the news, etc, so it's not the website that's causing them.

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