Hi hermann,
in the latest revision there are now the app specific keystrokes defined in the settings dialog under "keyboard. If I remember correctly, there's the possibility of redefining them by pressing backspace when placed on the key-combination, and then simply pressing the new keystroke. If this is right, then I couldn't succeed in redefining the toggle of structural navigation on/off in FF3.
The way I've been testing the redefinition of application specific key bindings is by Tabbing/Arrowing to the key definition I want to change in the key bindings list. Then I right arrow one column and press Space. I then type the new key binding (complete with modifier - Insert and "x" together for example) then press Return. Move to a new row in the list and repeat for other keys and press the Apply button to get them written out. Note, as reported in the comments of bug #433146 http://bugzilla.gnome.org/show_bug.cgi?id=433146 the one remaining problem left with the application specific settings in Orca is getting those new key bindings to immediately take effect in the running application. The workaround for now is to quit Orca and restart it. Hopefully that will be fixed soon.