I've found the reason! I suddenly wondered what would happen if I tried Firefox's Zoom Out keyboard shortcut ("Ctrl"+"-")... Ha!
I must have somehow pressed the Ctrl key and the "+" key when I was messing around with Compiz and KDE Desktop Effects a few days ago, and zoomed in Firefox on the SL forums page without realising it. Doh!
But why the problem only manifested itself on the SL Forums Web pages I don't know, unless Firefox remembers Zoom In/Out commands on a per-page basis? Anyway, egg on face and problem resolved.