What security? This is about write-only access and it already exists, just not for all media types.

So a website can inject crap into my clipboard? No thanks.

That is your opinion and your problem and there's an about:config option to disable it.

about:config isn't intuitive for users. It should be prominent under the main settings menu under privacy/permissions

Why? Why would a harmless feature that the vast majority of people want have to have its own "prominent" setting. I'd argue the exact opposite - that making it prominent would lead people to disable it without knowing the consequences and then think their browser is broken: "Oh, I don't want sites to access my clipboard, that sounds scary", then two months later "this browser sucks! When I copy my selection from Google Docs it strips all formatting! It works in Chrome, Firefox sucks!"

Anyways, this is far more time than I'm willing to spend talking to someone who thinks their PoV is the only correct one (I just now recognize your username from a few other threads under this post). Cheeers!