Is HN scrolling slow/jagged on Firefox for you?

Yes.

I just upgraded and firefox scrolling seems weird. It is freezing for 5-10s every few minutes.

Very obvious when I'll using the mouse-wheel to scroll down longs pages like HN

I've only used Firefox in the past couple of decades and the only site that does to me what you describe is LinkedIn. I can scroll about 2 days of updates before it lags to a crawl and then eventually breaks.

In the past there was paging issues for people logged in on HN when a thread would reach thousands of comments but team dang fixed that some time back.

Desktop or mobile? Just HN or others? I recall having this issue on a variety of sites a few months back. Their help pages had people experiencing the same issues but no solutions.

I think I downloaded FF Nightly and it wasn't an issue anymore.

Compared to most websites these days HN is about as lightweight as it gets. I would say if you are experiencing laggy page loads, the issue is most likely something else. (Maybe disable extensions and restart?)

Haven't seen that happen on HN ever even on pages with longer comment threads.

Just tried on Firefox on Windows and Firefox on Android. Both fine, as usual.

Nope. Everything fine as always.