Firefox question

[Timothy Murphy]

I'm running Fedora-22/KDE,
and am not sure if my query is a Fedora, KDE or Firefox query.

I notice that if I am viewing a page in Firefox,
pressing the down-key sometimes takes me down the page,
but sometimes does not.

Is this basically due to a difference in the HTML of the 2 pages?
Or is these some other explanation?
And is there any alternative to the down-key if it does not work?