Ah, I was wondering why my scrolling was getting "fling"ed so much on Nightly. It's kinda unusable because 2/3 of the time, when I lift my thumb off the screen, it gets interpreted as a fling. Maybe I'll get used to being very careful, but for now I'm glad to know there's a toggle.
There are still fling bugs to be fixed. :) Feel free to file a bug in Bugzilla that blocks bug 1448439. Perhaps you are seeing one of these known bugs: