Mozilla really ought to make work of explaining that permission to users, because I'm seeing this confusion a lot. The problem with it is that an addon either has no permission at all to modify a webpage (which includes necessities like detecting an image and setting a background around it), or gets full access to everything in the page ("all your data"), there is no in-between. Hope that clears it up for you, and I'm glad you took the time to check out the code and correct your own remark. Very cooperative and sensible of you, thanks.