Visit about:processes in the address bar (Shift+Esc) to get more details.

Does it still happen in Troubleshoot Mode?

yes it happens all the time. I have uninstalled 123 and re-installed 122.0.1, core usage is still high, but not high enough to crash the laptop, which has 36gb of ram, so I'm not giving up on it, but I will wait for next iteration of firefox and see if it handles ryzen any better.

Does it still happen with Developer Edition?

I am having similar problem except possibly my issue I am guessing is related to rendering ton of video data to Canvas. I have not traced it down yet to isolate but I am doing a ton of Canvas renders copying video over from WebRTC video onto canvas and yes it is intensive but not enough to crash the browser and slow the entire computer down. It works fine in Chrome and also the earlier versions of Firefox as well. I do a bit of graphics processing and then render that to the Canvas. So, I am assuming it is all the graphics manipulation and maybe the rendering to the canvas something is causing heavier CPU usage compared to previous FF versions.

-NEVER MIND- fixed my issue. This is not related to above issue..Sorry

Also, related to my reply above it works fine on Linux without much CPU. Maybe it is not efficiently using media decoders, or it is rendering slower.. not sure.. In Linux it seemed to utilize Media decoder and in Windows I never saw that pop up. I'll have to look at previous versions on about:processes

-NEVER MIND- fixed my issue. This is not related to above issue..Sorry