If you are unable to scroll vertically or horizontally using the arrow keys on your keyboard, you may have accidentally enabled Caret Browsing.

Caret Browsing is enabled/disabled by pressing F7 and allows you to navigate a web page just as you would in a word processor. A caret appears somewhere in the document and can be moved with the arrow keys and used to select text and click links using only the keyboard. When pressing F7 while Caret Browsing is disabled, you will get a dialog that looks like this:

If you checked Do not show me this dialog box again, then Firefox will enable Caret Browsing without prompting for permission. To reset this to the default behavior, do the following:

1. In the Location bar, type about:config and press EnterReturn.

   • The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise!, to continue to the about:config page.
2. Search for the preference called accessibility.warn_on_browsewithcaret.
3. Double-click on the key to set its value to true

Based on information from Scrolling with arrow keys no longer works (mozillaZine KB)