I had this problem in Edge with a RX6600 and it was real stutters in the image it's was not just Youtube stats. What fixed it was going to edge://flags and enabling:
Media Foundation Site Based Experimentation for Windows 10
If enabled, Microsoft Edge will use Media Foundation for media playback for a constrained set of sites. This feature works independent of the Media Foundation playback for Windows feature. The set of test sites is controlled via server configuration from Microsoft. This feature requires Windows 10+. – Windows
What is a bummer is that this a custom setting which can be reset with any Edge update but for now it works for me.
I haven't used Firefox for years but here is couple of settings in about:config (just type this in the address bar) you can try in every combination.
media.windows-media-foundation.enabled (maybe just put enabled if it is not)
media.directshow.enabled
You could also try to mess with the hardware acceleration setting in Firefox.
EDIT: Just came into mind you could try to enable AMD Performance overlay (ctrl+shift+o) and see if that helps with the dropped frames. It keeps the gpu "more awake" so it might make difference.
