I'd start by disabling all add-ons and check if that helps. If not, there's something wrong with Firefox itself.

If disabling them fixed the issue, I'd continue with a binary search to see if there's a specific addon causing issues. I.e. enable only half the add-ons and re-check. Then continue enabling/disabling add-ons until you know which addon causes the issue.

That said, it could be the combined load, or a combination of add-ons, which is significantly harder to fix.

Could be any number of things. Possibly a bad plug in or combination of plug-ins. Could be your new video card didn't jive with your installation of Firefox.

Run Firefox in safe mode and see if the performance improves. If it does, you likely have a plugin causing the issue. From there, disable all of them and reenable them one at a time to see if the problem comes back.

Disable and reenable hardware acceleration.

Disable ipv6 This is an issue I ran into earlier this year.

Clear cache and cookies

And last resort, Uninstall Firefox and all profiles and settings and reinstall a fresh copy. (after syncing your account, obviously)

What are your 25 extensions?

I found youtube videos unbearable as well and I found the culprit to be ambient mode in the video settings. I ended up adding a rule in ublock origin to delete the ambient mode stuff regardless of the setting just to make sure ambient mode is disabled in private windows etc.

@Yoru system specs? What add-ons are installed? What video res, 720, 1080, 4k?

25 addons vs stock chrome, I wonder what the problem could be?

must be because of lack of hardware acceleration. whats the cpu?

As they said start disabling every add-on. If this doesn't solve it, it may be an issue with hardware acceleration. I remember some installations where Firefox wouldn't decode video on the graphics card by default. Do a duckduckgo 'hardware acceleration \ Firefox' and see if it works.

Edit: Also codecs, maybe something with the h265 codecs

Try frontend alternatives like piped or invidious

What OS and device are you running on?

What extensions do you have running?

This is unlikely to be a simple Firefox problem. Try running Firefox in Safe mode and see if you have problems then (go into the main menu and then help to find the option to run without add-ons).

If the videos play well then it's your extensions. If the videos do it play well then it's drivers or the set up of your device.

I've only had problems with Firefox playing videos on a Raspberry Pi in the past due to a bug with drivers for video acceleration not working proberly. I had the problem with Pi OS but not a full Linux distro on the same device.