an IP editor at en.wp reports:
"At any given page, as long the edit box is selected (with the cursor blinking) pressing 'page down' or 'page up' takes you to the end of the page. Firefox 23.0, Linux Mint."
The example page they gave was [[Jet Lee]]
Using Firefox 23 on Xubuntu linux with the monobook skin I am unable to duplicate that behaviour, but:
At [[Jet Lee]] and [[User:Thryduulf/sandbox2]]: pressing page down or page up works as expected once but then does nothing. I then click anywhere in the body and again one of them works once, and then not until I click. However, once I've viewed the end of the page it expected.
At [[User:Thryduulf/sandbox]] and [[Timbuktu]] it works as expected from the start.
At [[Nigeria]] I saw the same behaviour as at [[Jet Lee]] but when I returned to the top of the page it went back to working only once.
At [[Great Balls of Fire]] page down worked once, then didn't. I clicked and it worked as expected subsequently.
At [[Malvern Link]] I saw the same as at [[Nigeria]], but after pressing page up both page up and page down then worked as expected.
I'm struggling to see any pattern.
Bug 33047 was suggested as having potential relevance, but that was fixed in October 2012 so my gut feeling is that its unlikely.