open slashdot.org in Firefox 3.5.3 on Windows XP and Debian Lenny. Drag the thumb down to the current bottom of the page. When the 10 -> 20 story reload happens, the scrollbar remains in the same place, but the page viewport moves to a new location relative to the now 20-story page.
This means that I scroll down to read a story, and before I can scan the headline, I'm several stories further down. Usually looking at stories I've already read.
Dragging a scrollbar thumb is normal behavior; Slashdot should not break this.