Today we’ve applied a minor update that seems to improve this situation — it looks like the issue is with how Firefox handles position: sticky; on avatars in posts as you scroll… the browser seemed to try to jump up to include the avatar sometimes when you try to scroll past it.
This specifically seemed to be an issue with higher refresh rates (I was able to reproduce it on a Macbook with “ProMotion”)… so it seems to be a very specific Firefox bug.
The workaround was to add overflow-anchor: none;
After updating Discourse, please let us know if you continue to experience the issue.