While I initially thought I had gone crazy or were still dreaming, my fear was true – all my keyboard page navigation had stopped working in Firefox (3.6.3). I tried to move onto to the next by pressing space or page down, my hands trembled as I discovered that neither arrow keys or home / end keys worked. The friendly internet suggested that I should turn Caret Browsing (F7) off (.. so try that if you’re in this situation), but that just told me that I were turning Caret Browsing on (with a proper warning). No luck.
After a bit of tinkering I disabled all my add-ons (previously known as extensions), and voilá, everything worked as it were supposed to! Further problem hunting revealed the culprit: The Del.icio.us Bookmarks Add-on. Curse you, del.icio.us! Disabling the Add-on and re-enabling all the other addons solved the issue, and I’m now happily browsing the internet one page size at the time again.