[WebExt] Ad parsing in dynamic Iframes

[cqx931]

This is actually an historical issue with dynamic Iframes in Firefox, see https://github.com/dhowe/AdNauseam/issues/627 I think we should better fix this in the new webExt release

Test case The ad within the dynamic iframe is not parsed

According to the note here: https://github.com/dhowe/AdNauseam/blob/master/src/js/contentscript.js#L1205 Is it true that content script would be automatically injected to dynamically-created iframes in firefox? @dhowe