[acqq]

Slowdown on Facebook and Youtube up to getting stuck (exactly what I talk about) was reported in some form at least since February this year, e.g. this Reddit:

https://www.reddit.com/r/firefox/comments/5sikxt/firefox_unb...

I've found that link after my GF complained many times, and I've seen the CPU at more than 90% (40% in Kernel) and Firefox stuck after only her visiting Facebook and scrolling down. It was hard even to kill the Firefox. I saw bugzilla entries too. Some quadratic loops for every element, triggering recalculating "everything" even if the result shouldn't change were mentioned if I remember.

What I claim is that on 56.0.2 (the most recent version until tomorrow) is still not better, FF getting stuck and CPU being fully stressed, and that the "subscriptions" can't be the only issue (which is the only scenario that the developers accused if I remember correctly the bugzilla conversations). The computer seeing this at almost 100% CPU and Firefox remaining unresponsive has only two cores, AMD CPU. If it's accidentally "better" on 16-core machine of some developer who doesn't use the mentioned sites anyway, my girlfriend can't change it.

Of course nobody is going to give FF developers her own Facebook access for them to reproduce it.

As for my GF the most of the internet are Facebook and Youtube, I suspect she'll really have to switch to Chrome if it continues. She already did "refresh Firefox" or however it is called now more than once. No change. And she is certainly not an outlier (and regarding the slowdown I see, I type these words on Firefox 57 (I downloaded it after reading other comments about the FTP availability, I see "Stylo true (enabled by default)") on 4 core / HT Intel and typing these last lines is also quite unresponsive and my notebook fan started at maximum even if unsurprisingly for 8 virtual cores I don't see much CPU use on the indicator -- something is still obviously wrong even on the very light site like HN and on the more powerful machine -- I guarantee you I see this only in Firefox and when I use native programming editors everything flies all the time).

[pcwalton]

If it is indeed style recalculation that is causing the problem, then Quantum (57) may well solve the issue, as Stylo both improves the dynamic restyling behavior (the style sharing optimization is more flexible) and accelerates the slow case in which restyling needs to occur from scratch (via parallelism). I can't promise anything without being able to reproduce, of course.

Naturally, like all browser engines, Gecko takes performance on popular sites such as Facebook very seriously.