1) Do some pages scroll only a half page when eone presses the space
bar? If so, is that the only time? What about when clicking with the
vertical scroll bar? Or pressing the Page Down key.

Is this dependant on what webpage I'm looking at; is it set in the
page's code?

2) PARTLY SOLVED: Often when I write a post it helps me find the right
search words, and maybe it did for this question, "stop automatic
reloading" That gave me
https://www.google.com/search?client=firefox-b-1-d&q=stop+automatic+reloading+firefox
which I haven't read much of yet.

Is there a problem with turning off automatic reloading? There must be
a reason they have it, not just to annoy me, so what will I lose if I
turn it off.

Do pages reload even when not requested. I know that jpg files etc.
continue to load and if they show on the screen they push any text that
I'm reading. I understand that and I wonder if I should pay extra for
faster FIOS/fiber optic.

But I'm referring to cases when the very top of the page shows for a
short time. This seems like a waste of my time when the computer is
running slow, which is fairly often. And the problem is compounded
because it doesn't always reposition itself where it was when it started
this futile exercise.

Is there a way to stop automatic reloading? Is this a Firefox thing
or do most browsers do it? Do any not do it?