In your "about:config" find "media.webm.enabled" and set it to "False" then restart. If your are on windowes then make sure you update your video card drivers direct from the manu, like nVidia, AMA or Intel, they have all released me ones
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A couple tabs? get a realt work load for ~30 tabs going and Firefox will always be way less then Chrome, every tab you open on Chrome eats ram, where on Firefox the inititial ram cost is greater (as the multiprocessing is for different parts, actions and rendering of the browser) Hence why there is not hiccups as theres never a process waiting/relying on the other to finish. That is what make Firefox so much more faster then Chrome now...
Tried what you said....problems with hd video remain...