If an add-on is modifying contents of pages it shouldn’t or of the clipboard when it shouldn’t, you would have to give it explicit permission at install time, i.e. “This extension can: Read and Modify Data on all sites you visit: Read and Modify contents of the clipboard.”
Obviously a simple URL redirector for wikipedia requesting access to this data is absurd and would be an immediate red flag. The reason this very thing doesn’t happen more often, is because frankly you’d have to be so computer illiterate to get to that stage that it is much easier to just phish you with basic Facebook profile info for much greater gains.
This is also the reason most “hacks” nowadays are either supply-side or phishing, shit is just too secure, no fun. We should bring back ActiveX.