It is possible to change the size of icons on GTK buttons, for example with this code in ~/.gtkrc-2.0: gtk-icon-sizes = "gtk-button=16,16" There is talk to make this size the default in GNOME 2.24. Currently mozilla assumes buttons should have 24x24 pixel icons. It would be nice if firefox could respect this setting for the bottom buttons in dialogs at least.
This is fixed in Fx now. If I find any place where it is still hardcoded, I'll open a new bug. Thanks!