How do I find certain objects like AD's with HTML or JS?

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Javascript

HTML

Firefox

extension

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So AD block is a AD remover well AD block finds the ADs and removes the lines of code that show the ADs and that is where I need your help.
I am trying to make an extension but I am not releasing the name or the extension till I am ready.

Now the extension needs to find the AD's codes and replace it with its own code when/while the page is loading or after it is done, now I am good at coding but not at JS and this is why I need your help I want to find all the ads and replace them with my own code now if anyone can help I will be greatly appreciated.