I found that the page up/down keys stopped working for me in Firefox. Searching around I discovered that caret browsing mode had been turned on and it’s had a bug since 2003. Caret browsing mode is not supposed to alter the page keys but some bug in caret mode breaks the page keys operation.
The fix is to turn off caret browsing mode by pressing F7.
You can also turn off the mode with this setting, “Tools – Options – Advanced – General / Accessibility – “Always use the cursor keys to navigate within pages””.