I was using a Core2 Duo with 4GB as my main home computer until about a year ago, and while some sites made it run slowly (mostly recipe sites and others with a lot of video ads) it was certainly still usable. The trick was to keep the browser down below 10 or so tabs and close it when I needed to use some other heavy piece of software.
Interesting, maybe it actually is Firefox being slower than chromium, but incidentally a few days ago I was trying to make an old core 2 duo laptop a browser-only machine for a friend's kid, and with debian 12 and Firefox, the start page of YouTube with no video playing had both cores hover at around 70%. The page worked but was notably slow. Videos played smoothly. So I guess if you don't mind the fans spinning constantly it's usable, but eh...
But as said, that was on the landing page with no video playing. What is it even doing there that can load up the CPU like that? I was rather surprised then that playback was actually smooth at least at 480p. Otoh I doubt core2duo had any sort of hardware acceleration for video decoding, but don't quote me on that...
> I doubt core2duo had any sort of hardware acceleration for video decoding
That would be the job of the GPU anyway, but H.264 decoding was pretty much standard back then.
That's why plugins like h264ify[0] exist for old hardware.
Make sure to use an ad blocker and turn off that weird Ambient mode graphical effect that just kills framerate. Videos should play fine after that on a Core 2 Duo if you have hardware accelerated decoding enabled.
