Does anyone know how to fix this popup?
This is because when I try to access my webpage, the popup appears.
So, anyone knows the solution?
I was first going to say that it's probably some activex spy/malware (at least a google search indicated that), but since you seem to be using Firefox, it might be something else. On which page are you getting the popup?
By the way, do you get the same popup on other websites?
Looks to me your site connects to advertlets.com which the word "advert" might mean "advertising" for short ,which may be sending pop-ups to/through your site. I can't seem to load their site, it is very slow. Maybe if you remove the script it might fix it.
I've already seen several cases where a counter script loaded a popup or even a virus, so yes that definitely could be the problem.