When I switched to Debian those years ago, X apps done from a su’d user didn’t work. I in fact decided that they don’t work, period, and for a good reason, so I stopped using them.
Today, I installed FF 2.0, and really wanted it to not use (read: mess up) my default profile. Despite this, I didn’t want it to ask me which profile to use whenever I started it. I know, I’m lazy that way. So the “su doesn’t do graphical” issue reoccured. And this time I decided that I have a good enough reason to look into it.
Update: gksu basically fixes the issue. A launcher with ‘gksu -u testy /usr/local/lib/firefox-2.0b1/firefox’ is enough. gksu is a GNOME application, but has equivalents for KDE, at least.