Replying to Ryan’s post…
And I’ll just quote that comment here:
Firefox-3 (now that the patch is applied) will behave like Gtk, i.e. it will read the font dpi from any standards-compliant XSETTINGS manager. If Gtk programs have the right font dpi on your system, then so will firefox.
As for the gconf key, it won’t help you unless you are also running gnome-settings-daemon (the default XSETTINGS manager for the Gnome desktop).
Let’s not spread unnecessary false information, shall we?