Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

Today I only see around 33% CPU usage, but yesterday I observed around 100% and the fans increased their speed. I believe it's due to the video in background. I find Firefox slower each version.

Is there a problem with any non-video pages?

See if there are updates for your graphics drivers https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

I was using nvidia's but then I got graphic artifacts (screen not well refreshed) after resuming from sleep.

So I tried the nouveau drivers and those artifacts vanished. But later I observed huge CPU load which would freeze the system until a hard reboot was done. So I was advised to go back to nvidia's.

In troubleshoot mode, this site seems to occupy 70% (web content process) + 30% (firefox process) but after some time is drops to 25+25.

I don't remember seeing a similar behavior in other sites. I assumed you would also observe this if you accessed this site.

I'll keep this under observation, thanks.

Can you post screenshots showing the issue? Will just opening the site cause the issue or are you doing other things? I'm not seeing any spikes on my pc.

Operating System: openSUSE Tumbleweed 20210723 KDE Plasma Version: 5.22.3 KDE Frameworks Version: 5.84.0 Qt Version: 5.15.2 Kernel Version: 5.13.2-1-default (64-bit) Graphics Platform: X11 Processors: 8 × Intel® Core™ i7-4810MQ CPU @ 2.80GHz Memory: 31.0 GiB of RAM Graphics Processor: Mesa DRI Intel® HD Graphics 4600

Not sure if this screenshot helps.

Operating System: Linux Mint Cinnamon Kernel Version: 5.4.0-80-generic Graphics Platform: Xorg?

processor : 7 vendor_id : GenuineIntel cpu family : 6 model : 26 model name : Intel(R) Xeon(R) CPU W3540 @ 2.93GHz

MemTotal: 6069048 kB MemFree: 657904 kB MemAvailable: 3297916 kB

28:00.0 VGA compatible controller: NVIDIA Corporation G98 [Quadro NVS 295] (rev a1)

This is a relatively old machine.