I don't use an iPad but here on Android Firefox's built-in tracking protection zaps most Google Adsense adverts. I see it's available for Firefox on iOS. Is it turned on?
https://support.mozilla.org/en-US/kb/tracking-protection-ios

And is it really a drop-down - i.e. it overlays the webpage content? If I disable tracking protection I see wide narrow adverts inserted between the content.

Firefox for iOS uses the same underlying engine as Safari so if something has infected that it will happen on both.