on web pages such as
http://forums.mozillazine.org/search.php?search...
http://forums.mozillazine.org/viewforum.php?f=3...
I'd like to go to the Next Page via

Alt - [ Page Down ]
or
Shift - [ Page Down ]

as an alternative to using the mouse;
because when I'm Paging Down, I'm not using the mouse

Try this: http://userscripts.org/scripts/show/3152
It doesn't do exactly what you want, but it adds the access keys that should be there in the first place.
It is a good script, if I have time I might modify it to do more.

thanks; i already have that
:-)