Thanks. I got a 2DS on 11.1, I will be fine. Thanks for the link.The original FIRM files are available on the 3DS guide as magnet links. Check the "Installing boot9strap (hardmod)" section.
Do note that if the FIRM you install doesn't match the OS installed on CTR NAND, it might not boot. You may be able to boot into recovery mode to do a system update, though this will bring you to 11.4. If you currently have B9S+11.4, installing the original 11.4 FIRM will make it impossible to reinstall B9S unless you have DSiWareHax or a hardmod.
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Also, how would we install it? Use B9S to install it?You could install the original Nintendo FIRM into the FIRM partitions, but I'm not sure why you'd want to do that...
Note that while this wasn't safe to do on New3DS with A9LH, it should be safe with B9S because the secret sector (sector 0x96) is restored as part of SafeB9SInstaller.