This functionality has been baked in Firefox for a long time and I use it quite often. Not only makes the ads and other clutter disappear, but it also removes most of the "disable your adblock" unclosable popups.
It is definitely one of the super-nice things to have baked in straight in the browser, and probably one of the major differences I see between Chrome and Firefox in day-to-day usage. I just wish that somehow it was persistent for certain rules, but I can see how that is a slippery slope.
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