Scrolling with arrow keys no longer works
From MozillaZine Knowledge Base
This article describes how to fix a problem in Firefox, Mozilla Suite or SeaMonkey, where you are not able to scroll through web pages when you press the Arrow keys, Space Bar, Home, or End keys. Such problems may be caused by having either Caret Browsing turned on or having the Number Lock key engaged. If the scroll bar is gone, see Scrollbar gone instead.
When Caret Browsing is on, web pages act like word processor documents, with a blinking vertical bar that shows you where you are in the document. You may have inadvertently enabled Caret Browsing, which is toggled on and off by pressing the F7 key. To turn Caret Browsing off, press F7.  Note: On laptops and other computers with compact keyboards, the "fn + F7" key combination turns Caret Browsing on and off.
If you see this dialog and you do not want to turn on Caret Browsing, click No.
If you selected "Do not show this dialog again", you can bring back this warning dialog by setting the preference accessibility.warn_on_browsewithcaret to true:
To reduce such problems in the future, you can add the following in your user.js file so that every session begins with Caret Browsing off and the warning dialog enabled.
// Get SpaceBar, PageUp, PageDown, Home, End to work properly if F7 hit accidentally user_pref("accessibility.browsewithcaret", false); user_pref("accessibility.warn_on_browsewithcaret", true);
When the "NumLk" key is active any keys that share the numeric keypad will be inaccessible while Number Lock is engaged.