Please do the following:

1. Click Help | Restart with Add-ons disabled.
   
   
2. In the next menu which appears, checkmark: "Reset toolbars and Controls".
   
   
3. Finally, click the button called "Make changes and restart".

Sounds like this bug:

• Bug 640587 - Page up/down behaviour changes to "page back/forward"

Thanks guys, did what you suggested and it works somewhat, but after a couple of days, firefox went back to doing the same thing