The issue is trying to run a video in firefox. Modern web browsers consume a lot of resources. Also they don’t use your hardware efficently for video playing. You need to take some time to set up a native video player application to play youtube videos. This better uses the SBC hardware acceleration without wasting precious resources.
I have revived lots of old PCs over the years. Giving them a new lease on life with up to date linux operating systems for friends and family. I am also a big fan of watching youtube. The strategy I came up with to watch youtube on lower end aging systems is this:
Install the latest versions of smplayer, smtube, and mpv. Its important to use a distro that keeps these software repos up to date. Use smtube to select a youtube video, it sends it to smplayer which downloads the latest yt-dlp. If everything is up to date, it plays great.
I have a 15 year old laptop that was finally having a hard time running latest linux mint xfce. This week I got to work reviving it. I gave mx linux a shot as I liked ExplainingComputers review of the OS and thought it good fit for my use case. Installing these programs right from MX’s software repositiories was a breeze. Youtube played effortlessly! MX is pretty minimal and im sure most pis can run it okay, so give it a shot if you want a OS with up to date repos for these packages if youtube is one of your main concerns.
Once you get youtube videos playing, go into settings of smtube to change the web page from tonvid to a custom invidious instance. Pick one thats ideally from your country and that lets you register an account. That way you can import subscriptions and personalize stuff.
The only hiccup when using smtube to load an invidious site is this; the default language will be some foreign language. Make sure you know how to go to settings in invidious and change to english.