it's almost certainly not an issue of conflict with older version.
it's probably an issue with certain cases where the autoscroll in ff is not working due to something on the page confusing it (animated gif), or some input field thing (if you are in an input field then when sc sends keystrokes to simulate scrolling they dont work), or some other fluke.
so when it fails again see if you can identify what is confusing it and what makes it work when it works.
we still have some tweaking to go to make scrolling cap perfect, but with some help and good bug reports, we'll get there