nsITextScroll in Firefox 3.0

 Scroll the view up or down by aNumLines lines. positive
 values move down in the view. Prevents scrolling off the
 end of the view.
 @param numLines number of lines to scroll the view by

 Scroll the view up or down by numPages pages. a page
 is considered to be the amount displayed by the clip view.
 positive values move down in the view. Prevents scrolling
 off the end of the view.
 @param numPages number of pages to scroll the view by