Most of the time the down arrow on the scroll bar is missing or occupies the same space as the page expansion square in the lower right hand corner. This happens whether the up scrolling arrow is at the bottom or the top. It happens with all pages on Firefox and most pages from search engines. Sometimes it is there. It is on this page and some linked pages.
try resetting the scrollbar arrow location in system preferences->appearance. Just change it from together to at top and bottom and see if it clears things up.
I donernst got a solution for this years ago, but for those who have this problem today and found this by googling for a solution. I found one when I was having this problem with Foxfire. By going to View and choosing a new text size I was able to get my down scroll bar to reappear. Not elegant but a solution...