It might be using a 'fuck-with-the-scrolling' JS library, like a few poor sites do here and there. Does it get better if you disable JS?
Really not sure what does that. Not using any JS scrolling library. Perhaps a performance hit from using backdrop-filter on the table of contents section for a background blur effect.
Are you on iOS or on Android? Are you using any ad filtering extension?
I checked on both FF and FF Focus on iOS and got regular scrolling behavior.
I'm using FF on android and had no issues scrolling.