could you confirm those points :
1/ Your are not using xbmc directly from gdm, but lanch it in Gnome, KDE, etc...
(firefox need a user interface)
2/ the three paths you give me, permit to launch firefox in a shell
3/ put the file below in your home directory, edit the firefox command in it, and browse to this file with the webbrowser settings (the file needs to be executable)
file : http://dl.free.fr/vYRBRguF4
If it doesn't resolve your problem, give me your log please, perhaps something is broken before your launch the bookmark
OK I will try this tonight. thanks