I am fairly sure your problem lies here:

In the log, when you have made the kernel selection and the kernel image is loaded, you should see something like you have when starting harmattan:

Oct 6 19:34:39 (none) user.notice root: Started animated OS selection menu
Oct 6 19:34:46 (none) user.notice root: Selecting Harmattan OS, running kernel /boot/Harmattan/boot/zImage_2.6.32.54-openmode_l2fix
Oct 6 19:34:47 (none) user.notice root: Loading kernel /boot/Harmattan/boot/zImage_2.6.32.54-openmode_l2fix
Oct 6 19:34:47 (none) user.notice root: kexec_load() successful
Oct 6 19:34:47 (none) user.notice root: Restarting to selected OS

However, you got this:

Oct 6 19:40:14 (none) user.notice root: Started animated OS selection menu
Oct 6 19:40:20 (none) user.notice root: Selecting Firefox OS, running kernel
Oct 6 19:40:21 (none) user.notice root: Cannot load Firefox kernel
Oct 6 19:40:21 (none) user.notice root: Boot OS/kernel selection failed

Now, this looks to me like the kernel name string is undefined here. Either the problem is in the selection script or your config, but as you can boot it when used as the first OS, then it sure points to a menu script problem.