First, click on the title bar. (should say FBReader.) Go to the Delay between Scrollings, msecs. You should be able to set this to a very high number (5000 is the max.) Mine's at 250, and I never notice it auto-scrolling. Don't know if this helps.
I assume you want autoscrolling. You could try asking on FBReader's Google group. The developers are very responsive, and will at least tell you when it might be available.
FBReader does have three scrolling modes on the 770 (large, small and finger tap), all are customizable and the first two can be mapped to any keys. I find using the top rocker button for page advance very comfortable, but this does end up being a lot of key presses per book.
