With either Firefox or IE, if you say “scroll down,” you are taken to the very bottom of the page. Especially with Firefox, I don’t know that there’s a fix for this. Therefore, I just say “page down” or “page up.” Alternately, you can say “move down/up [some number].” “Go down/up [some number]” works also. But to me the movement is too jerky. More often, I just use the Autoscrolling Bookmarklet. (For Firefox, that is; I don’t use IE.) That way, I don’t go hoarse.
However, if you are an IE user, Nuance wrote a perk for you, which they did not extend to the evil stubborn Firefox users. If you use IE, you have auto scrolling commands built in. Look in your Command Browser. (The auto scrolling was not enough to make me switch back to IE for Dragon use; I would rather tinker with settings and add-ons and push for accessibility in order to have a choice of what I will use. Monopolies, even if unintentional, disgust me.)