I still have no idea what caused the bug, but here's how I fixed it:I removed the Firefox icon from favorites.
At the login screen, I chose the normal Gnome desktop instead of the netbook launcher. (I possibly could have omitted this)
I started Alacarte, the Gnome-menu-customization tool. (Turns out this one also works for the netbook launcher, i should have figured this out earlier.)
I created a new menu item, an application launcher to start "/usr/bin/firefox". This is what worked from the shell.
Back on the netbook desktop, I put the new icon back into my favorites.
No problems so far.