Yeah, browser plug-ins are a great idea, but my grandma's on internet explorer, and I don't even know if she goes anywhere except Facebook, ya know? For the life of me, I can't figure out how to solve the problem of the 65+ demographic.
With how most browsers are engineered, I don't think there's a need for putting it into the workings of a browser. It's a perfect use-case for browser extensions or releasing something similar to Firefox ESR with a bundle of pre-installed and pre-configured extensions (it's essentially the idea behind the TOR Browser, at least its early releases). Plus consider all the outcry resulting from making it opt-out. Libel/slander, "who watches the watchers", accusations of censorship, genuine false-positives, media outlets using the tool against competitors, and who knows what kind of legal implications. Those always follow even in much more benign changes when it comes to content flagging.