Unable to scroll browser pages with arrows and PgUp PgDn keys

I don’t know what keys I press sometimes and the pages of my (Firefox) browser can not be scrolled with the arrows and page up and down keyboard buttons.

A lot of users solve this problem using the F7 key (a toggle key to enable and disable selection of non-editable on line text with the keyboard). This doesn’t work in my case. I can not even move the cursor with the keyboard to select text.

I’ve tried also all the F keys and none worked, nor the FN key, ScrollLock, NumLock, the Alts or Ctrls.

If you face the same problem the only solution I’ve found so far is to restart the browser. In case you are in the middle of some work involving certain pages and tabs to be open, backup first your session (by going to Tools > Session Manager), then restart Firefox.