I've recently had this problem where videos on youtube will play audio perfectly, but the video will lag every single time. I looked it up and people said it was because the cache was full, so I deleted my cache, cookies, history, everything, yet I'm still getting this problem. I'm using Firefox, and the videos play a little better on Chrome (I guess since it handles HTML5 better or some shit idk), but I'd rather keep using Firefox. Anyone else have this problem/know how to fix it?

since you mentioned HTML5 ... try installing Flash Player for Firefox.

Try reinstalling video card drivers

If you're on Windows 10, try Edge.