A question about a survey pop-up on Google

[Hubbaman]

Hi,
I have just had a couple of pop-ups in Firefox after doing searches on Google. The hits are listed, and suddenly the browser window goes all grey, and shows a message saying something like "Win an iPad, take part in our survey" and a button to click ok. (Perhaps it's not a pop-up as such, but more of an overlay ad.) The text was in my local language. I simply closed the window to get rid of it.

I don't remember seeing this before, and I was a bit worried as this happened right after I installed the latest version of CCleaner. I think I also had done a cleanup with CCleaner and rebooted before it happened, if that could have anything to do with it.

I have scanned my computer with Avast and Malwarebytes Anti-Malware. Nothing suspicious.

Googling "Google survey pop-up" etc doesn't really make me any wiser, so I thought I'd ask you. Do you know what this is?

Thanks.

Edit: One address showing in my log says this was celluway.com/...

Edit 2: Sorry if this topic doesn't belong in this forum, it's just that I have gotten so much good advice here before.

[Para-Noid]

Since you have Firefox do you use "AdBlock Plus"?
The pop-up is not security related it is browser related.
With AdBlock Plus "right" click then select. Or simply click on the "x" or "close" button. 

[Hubbaman]

Thank you for the tip. I will give that a try if the problem continues. I'm normally not very bothered by advertising, so I haven't felt the need to block ads.

I was just surprised to see that kind of behaviour on google.com, so I was worried it might be something suspicious.