All of the old themes CSS are now user maintained
. If you have suggestions for CSS changes to them, you can present a fix there.
As for the default theme, the body background has no images and is only one color and still a brief white-out can exist, typically (if not exclusively) at the start of a session in Firefox. IE9 does not seem to have any such issue. There's nothing wrong that I can see.
I actually get this on the simeplemachines wiki and forum
(both running off an SMF portal) I would guess this is something related to how SMF and current templates load now in firefox. If you figure anything out, let me know.
This link seems to do it every time for me on their site (I am logged in there w/ account if that makes a difference):http://wiki.simplemachines.org/smf/Bulletin_board_code