Ah hell I'll try it. I dumped my DS Lite's bios a while back but I've lost the files (I'm sure they're here somewhere). When you download a bios online you're also getting the profile, name, and stuff like that. So while you could download a bios, it's not customized. As far as I know it's also not possible to alter a firmware from desmume (unless they changed it).
DeSmuME should run just fine without bios/firmware files.
To help make it run faster you can enable dynamic recompilation which should massively speed it up.
What's the advantage of having the bios files anyway?
Some games uses the console information (such as Prof. Layton Series) to calculate something. So i can not import the Password from Desmume Save to 3DS one.
However the rewards are just some illustrations, and I finally got those from the walkthrough.. easily. So Problem solved for me.
In fact, BIOS emulation would infer LLE (Low-Level Emulation) and it is HLE-ed(High-Level ..) if without the BIOS.
HLE runs faster on PC and LLE runs almost accurate always. So in fact the bios file may lead to a speed decrease. That's all what i know about the DS BIOS for emulation.