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)
I always run with hardware acceleration off on all my Firefox machines, it seems to cause a lot of issues and the videos play just fine without it.
Depends on the setup. Disabling it on mine turns Firefox into a slideshow.
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.
What was the rule you added? I turn ambient mode and in-line playback off but Google is constantly resetting those switches.
I don't have access to my desktop computer at the moment but i believe it was this rule: https://old.reddit.com/r/youtube/comments/ykana1/how_do_i_disable_youtubes_ambient_mode_permanently/j9khxuh/
Thanks, I’ll give it a shot!
@Yoru system specs? What add-ons are installed? What video res, 720, 1080, 4k?
recently upgraded to an rx 6600 on a ryzen 5 5600 also 16 gigs of mem. here are the addons: 
Note that Dark Reader is insanely resource intensive.
(Also note that Disconnect has been unmaintained since before the pandemic and is redundant because UO exists.)
@Yoru Try clicking the gear in the menu bar, and turning off Ambient Mode. That lagged the crap out of my machine, but on a Intel iGPU.
The dark themes in Enhancer for Youtube also disable ambient mode.
25 addons vs stock chrome, I wonder what the problem could be?
the thing is it was running perfectly fine with 40+ addons, I guess it has smth to do with my new gpu? I'm outside rn I'll try the things the people recommend
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
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.
Try frontend alternatives like piped or invidious
unfortunately youtube isn't the only culprit. Firefox just seems laggy in general, the affect is multiplied in youtube and indivious doesn't 1080p or 60 fps :/
Does the same happen on a clean profile? (you can create a new one in about:profiles)
On my machine it doesn't even run properly after fresh installation and new profile and not one single page opened.
I only have four or five extensions and YouTube videos lag. It's Firefox. I'll still not switch to anything else, but I could imagine a clean Chrome without anything added and zero bookmarks purely as video platform.
i have over a dozen addons in firefox, several user scripts (including ones that run on yt). i never log in.
i can q-up a days-long playlist and it'll just go and go. no problems.
Can you also watch twitch at 1080p without lag? Maybe you don't watch it, but I always had problems with that. Chrome works perfectly on twitch but not Firefox
Firefox plays 1080p twitch streams just fine for me on several machines.
i can watch 1080p content just fine on my geriatric pc (1st gen lynnfield), and even do h264 encoding in realtime or faster (sw ffmpeg), but discord and twitch just flat-out suck on it.
Hmmm, maybe my PC just sucks :'/
It could be many things.
Do you have a lot of extensions enabled?
Is your Firefox updated?
Is xmp or docp enabled so your ram is running at the right speeds?
Is hardware acceleration enabled/disabled in firefox?
Have you tried a fresh firefox profile/install? you could try downloading a beta/dev version and testing with that.
I have like 9 extensions, Firefox is always updated. I don't know but I have 8gb ddr2 ram (10 year old PC), I don't think I enabled hardware acceleration. I have not tried a new install.
I watch twitch at 1080p and YouTube at 4K with Firefox on Windows and Linux (same PC) without issue.
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