User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:60.0) Gecko/20100101 Firefox/60.0 Build ID: 20180219220041 Steps to reproduce: Opened a fresh FF Nightly Build, single tab opened to a Twitch stream at *only* 360p resolution. MacBook Pro 13", Early 2011 2.3 GHz Intel Core i5 8GB 1333 MHz DDR3 Intel HD Graphics 3000 512 MB Profile and video captured here: https://perfht.ml/2GsTtrt https://youtu.be/dgWX8SJc98I Actual results: CPU spikes to 150+% in v58, spikes to 140% and stays around 110-115% in v60 Nightly build, excessive heat and cpu fan stays at max RPM. Expected results: For reference, same stream opened in Safari is 20-40%, Chrome is 60-80%.

Hey Mike can you take a look at this profile when you get a chance?

There's a bunch of jank from React rendering components and constructing the page. There's also something painting and animating all of the time due to this GIF that's animating despite being hidden by some other UI: https://d3aqoihi2n8ty8.cloudfront.net/actions/cheer/light/animated/10000/1.gif Even still, I would wager that the vast majority of the CPU is going towards video decoding and rendering. That's my take on it, anyhow, based on my read of the profile.

Another one for your tracking bug, Dan.

Still having this issue. Any new information? Safari runs Twitch significantly better on my 2014 MBP (i7 and 16 GB RAM).

Just gave a quick try on macOS 10.11 / 61.0a1 (2018-04-25). I checked with Webrender enabled (WRe) and disabled (WRd). CPU ~50% in both cases, but also having a heat spike with my fans doing overtime. WRd: going up from ~2100 rpm to ~6200 (100%) and heat of CPUs and GPU from 55/60˚C => 80/85˚C WRe: going up from ~2100 rpm to ~5900 (90%) and heat of CPUs and GPU from 55/60˚C => 70/75˚C

Still seeing this on 62.0.3 (64-bit) on Model Name: MacBook Model Identifier: MacBook10,1 Processor Name: Intel Core i5 Processor Speed: 1.3 GHz Number of Processors: 1 Total Number of Cores: 2 L2 Cache (per Core): 256 KB L3 Cache: 4 MB Memory: 8 GB Firefox will regularly consume 110% cpu doing decode. Safari comparatively about 11% and is clearly leveraging GPU hardware decode.

Sorry, should have added this is on 10.13.6 with this GPU spec: Chipset Model: Intel HD Graphics 615 Type: GPU Bus: Built-In VRAM (Dynamic, Max): 1536 MB Vendor: Intel Device ID: 0x591e Revision ID: 0x0002 Metal: Supported, feature set macOS GPUFamily1 v3