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Until they provide a decent UI/UX to do that, you can use about:performance for now.

I agree, this might be "good enough" for now; thanks for the tip

Adding the addon 'The Great Suspender' helped me out in this area. Also disabling HTML5 autoplay.

HTML auto-play is absolutely infuriating. On desktop, I landed on some CNET article the other day and suddenly a video start playing (super loud, too). Why does CNET assume I want to watch this?

I don't know if there's an option somewhere in FF or Chrome to block this nonsense but I really hope that feature is implemented some day.

about:performance

That's useful, didn't know about that; but it doesn't show per-tab usage like Chrome's does (I think).