A place to discuss the news and latest developments on the open-source browser Firefox
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?
@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.
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!
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
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.
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.