The scroll speed is known to be overly fast on Firefox and this is a Firefox-specific issue.
If you do find the overall scrolling to be too fast, you can change the setting in the KDE System Settings under Touchpad (this will affect all apps equally). GNOME, as usual, is allergic to configuration settings and still doesn’t have that one, but there’s nothing we can do about that.
As for palm rejection, that is largely handled by the trackpad firmware and disabled in libinput for Apple machines since it was causing trouble. However, that issue may be obsolete due to other changes, so we could try re-enabling it. Firmware palm rejection seems to be quite sufficient for press-to-click mode but not tap-to-click mode (I’ve only heard complaints about tap-to-click, if you don’t need that you might want to turn it off).