User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_2) AppleWebKit/604.4.7 (KHTML, like Gecko) Version/11.0.2 Safari/604.4.7 Steps to reproduce: On MacOS 10.13 && Firefox 57 - open a new tab - load www.twitter.com - look in Activity Monitor, WebContent is drawing ~60% CPU even though no visible animations, it stays there indefinitely - look in Intel Power Gadget if you have it, GPU is drawing a lot of power. - if you scroll the page down until all the content is loaded the CPU drops to normal Do not see this behaviour on MacOS 10.12 or on Ubuntu Could be a CSS animation not throttled? There is a swirler at the bottom of the page & a sliding navbar Could be part of bug 1422090 Comment 37 (Issue 2)?

Can you reproduce the issue with Firefox 56?

Same issue occurs on FF56 & 57. If you were asking about servo/sylo I disabled it and tried again. No change. WebContent CPU twitter.com after loading but before scrolling FF56 ~60% FF57 servo ON ~60% FF57 servo OFF ~60% MacBookPro11,1 / MacOS 10.13.2 After I have scrolled down and triggered the swirler which loads new content, WebContent CPU drops to single digits and stays there Incidentally I tried Hiro's patched version which fixed theguardian.co.uk see bug 1419079 Comment 21 but it didn't fix it, so presumably a different issue. My guess is it's a throttling issue, maybe the swirler runs all the time offscreen??? I checked FF57 on Ubuntu VM again and again didn't see the issue

I think it's the animated gif spinner-rosetta-gray-32x32.gif running all the time. It's the loading spinner for more content at the page bottom. I set up a uBlock Origin filter to stop that gif loading and afterwards, CPU load was very low. Then I got rid of that uBO filter and reloaded twitter.com, back to 60% CPU. Then I set image.animation_mode to none and reloaded twitter.com, again CPU load was very low. I think the animated spinner gif runs all the time & uses CPU and GPU to do a ~40 FPS display refresh. Again possibly on MacOS 10.13 only, not on Ubuntu??? Might be Twitter's fault not Firefox's? But if so then why doesn't it occur on Ubuntu?

(In reply to Mark from comment #3) > I think it's the animated gif spinner-rosetta-gray-32x32.gif running all the > time. It's the loading spinner for more content at the page bottom. Bug 1409166?

I don't think so. bug 1409166 seems fixed in my current Nightly 2018-01-15 (~8% CPU) but bug 1425684 does not (~25% CPU).

That's right, bug 1409166 is for CSS animations/transitions, not for animating images.