If you're actually experiencing 100% cpu usage, post a profile using https://perf-html.io/ and I can try tell you why.

1) Pick a rMBP. Any one.

2) Set display options - "more space" under System Preferences.

3) Run Firefox. Load any website. No extensions; safe mode doesn't matter.

4) 100%+ CPU, fans kick up and run high, system gets hot and slows to a crawl.

Chrome and Safari run totally fine under the same conditions.

It's documented pretty thoroughly in the first Bugzilla link I posted; the second one is an offshoot where they're trying to solve this specific issue. If you do happen to work at Mozilla, would really appreciate your help escalating.

EDIT: I now have a perf file showing significant issues when clicking on a YouTube video. It gives an error when trying to upload it to the site, but if you contact me through email (in my HN profile), I can send it over.

2) My display options are set to "more space"

3) I loaded this hackernews thread.

4) Firefox and it's child processes are using <1% cpu.

If I aggressively scroll this page I can push the cpu usage up. We have a variety of fixes in the pipeline that will help with this (https://bugzilla.mozilla.org/show_bug.cgi?id=1429522, https://bugzilla.mozilla.org/show_bug.cgi?id=1265824). That being said, only 4.7% of our users are on MacOS so it's more difficult to justify prioritizing work specific to that platform.

If that second number is substantially higher (and I personally suspect it is, but I don't have data backing that up) I think it would be very reasonable for Mozilla to prioritize MacOS. Getting web developers to use firefox makes firefox work better for everyone because it makes more sites work well with firefox.

On that note, one of the key motivators behind WebRender (our new graphics backend) is that it provides hardware acceleration across all platforms, including mac (whereas our current Direct2D acceleration works only on Windows). So Mac graphics performance should generally improve once we get that shipped.

Draw from that what you may, but I bet that dynamic is happening in other places.

However, I would like to ask... 4.7% translates to how many millions of people?

Edit: Actually I just checked and the Windows 7 machine doesn't do it any more. I'm not sure if maybe a Firefox update fixed that here. I'll double check at work tomorrow too

Edit: For people who want to compare notes: High Sierra 10.13.6, Firefox 62.0.2, resolution is set to "Default for display" (which I think does mean it's doing interpolation of some sort). Automatic graphics switching is enabled. I don't have any trouble watching YouTube videos.

I'm not sure what to say. I'm sure the issue is real, but it seems like we're missing something in the triage. There has to be something else that makes a difference.

I've got a 15" "late 2013" rMBP, set display options to "more space." I've got a total of 45 tabs open between a couple different windows (I use the userChrome hack to re-enable multiple tab rows.) Cycled between tabs, refreshed, navigated around FB, GMail, etc. Can't get the CPU to do anything out of the ordinary.

Most developers in their early 20s seems plausible, but even there, I surveyed my team’s younger cohort, and it was less than 50%.

On the flip side, I've had a recurring bug in Chrome (until very recently) where for any long running session the Browser process will consume 100% of 1 core. I've had it across multiple machines and profiles, including a completely clean profile with no extensions - I've never managed to be able to nail down the common elements.

Why?

Firefox with one tab (with gmail opened) takes 20% CPU. (and a helper process "FirefoxCP Web Content", which takes 50% CPU, wow.)

Chrome with some 20 tabs, ~4-5% with all its processes (but a lot of ram, which I am fine with)

This is for a 2015 13-inch Macbook pro.

Facebook has also replaced forums for our industry (cell phone/computer repair), so it's how we find out about the latest trends and innovations. Yesterday I used a document posted on a private FB group to fix 3 2011 15" MacBook Pros with failing GPUs (previously considered "unfixable.")

For the industry and business I'm in, there's no substitute.

Private groups on FB can be more discerning about their membership, which is also what makes them interesting.

Many of the cell phone repair groups I'm in require you to have a repair business, so the groups don't get flooded with what they call "plz bro" posts. (i.e. "How do I fix an iPhone screen?" beginner posts.)

It also forms a tighter community; we pretty much all know each other on there, so we refer business back and forth a lot, and use Messenger heavily to buy, sell, and just chat about really obscure or weird repairs that come in the door.