My wife has an apple and it sort of mystifies me. The functions are almost the same as in Windows, but everything has a cute name. It's a mystery. Some setting is telling the pop up to center. Does Firefox have a version for Apple? I'd try that if available. If you can sometimes clear it, then it may be a codeing thing from Google. If Safari has settings for popups, you might look and see if it helps. Or, you may have downloaded some malware the puts up the ads, especially possible if the popup ads don't relate to your online activity or searches at the time.
The more I do, the less I accomplish.