Try setting fission.autostart = false on the about:config page to disable Fission. https://hacks.mozilla.org/2021/05/introducing-firefox-new-site-isolation-security-architecture/

WARNING: Changing preferences through this interface not officially supported Hidden settings edited using the about:config tool are explicitly not supported, which means that Mozilla makes no guarantees they will be supported in the future, or that Mozilla will fix them if they break. Mozilla does not test these preferences, and will not in the future. That includes security and performance testing which these preferences may affect.

[Warning added by moderator.]

Tried that but still the same. CPU up to 100% ...

Try going to about:performance to see what's causing it.

Not much to see there. Only the tabs and extensions that was running.

Go into Safe Mode to see if you have the same issues. https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode

In safe mode CPU usage is much lower, about 30%

Sounds like a add-on could be causing the issue. Try turning them back on 1 by 1.

I have several YouTube tabs and not seeing that issue. The laptop I'm on dual boots Win7 and openSUSE Tumbleweed.

Operating System: openSUSE Tumbleweed 20211213 KDE Plasma Version: 5.23.4 KDE Frameworks Version: 5.89.0 Qt Version: 5.15.2 Kernel Version: 5.15.7-1-default (64-bit) Graphics Platform: X11 Processors: 4 × Intel® Core™ i7-4810MQ CPU @ 2.80GHz Memory: 31.0 GiB of RAM Graphics Processor: Mesa DRI Intel® HD Graphics 4600

If I disable evey extension its still the same. Its only in safe mode the CPU usage is lower

I forgot, update to 95.0.1. What security software are you running?

I'm already on 95.0.1 I haven't installed any security software, only what windows is providing.

From about:performance, can you tell which tab is using the most memory?

The youtube tab uses most, 76,5MB In linux Mint, CPU usage is only 15-30% when watching youtube 1080p videos.....

And Chrome uses 15-20% CPU on the same videos ......

I recently started encountering such behaviour (fan spinning up on Firefox), WebExtensions process up to 100%. I see the following (see image). The Firefox Task Manager showing hardly any processor load, the WebExtensions process showing 100% load. It appears to start to happen when I have had one or more Video websites open (not only YouTube, also other streaming services).

It appears that it is sort of random whether the WebExtensions process is running "hot" or not. Loading video in multiple tabs seems to worsen the problem, but the 100% load is not always coming up.

In this case I had closed all Video Tabs after playing some YouTube and other videos. The WebExtensions tab was (and currently is) still 100%. Maybe some spinlock or other process quirk?

The only way to get rid of this is either to kill (-s 15) the "hot" process, thereby loosing the Extensions, or to close Firefox and restart, and hope for the best when not running Video... (Normally video decoding takes about 30% of CPU on one core).

I know this is not very precise info, but I hope it may serve to reproduce. Running Firefox on Debian Stable, 91.4.1esr (64-bit).

Best regards,

Tjeerd Pinkert