How to scroll in the Terminal app "top"?

I have only one suggestion: use htop instead.

sudo apt-get install htop

The point of the "TOP" program is to tell you which processes are using the most resources or are at the TOP of the list.

For a full process list use "ps":

ps aux

You can filter it with "grep":

ps aux | grep firefox

This is a bit old, but I'm finding that I can scroll top just fine in version 3.3.9 (Default on Ubuntu 14.04)

If you hold the down arrow, it'll look like you aren't scrolling, but just keep holding. For each repetition, I believe there is a marker that is moving down, row by row (not visible) and when it hits the bottom the window scrolls down.

Also the PageUp and PageDown keys work a bit faster.