I copied the required firmware files, then I load the core and check “Information -> Core” and everything looks fine to me:
However when I start the core (with no content) melonDS DS displays a message saying “The firmware file firmware.bin can’t be used in DSi mode”. And that’s correct because dsi_firmware.bin is supposed to be the DSi one.
But in “Core options -> System” I can’t change the “DSi Firmware” option. If I select it “firmware.bin” is the ONLY option:
Does this work for you? Could have the wrong dsi_firmware.bin file but unfortunately the wiki states that the md5 “varies” so it’s hard to tell. If your dsi_firmware.bin works for you and it’s not against the rules to post just the crc32 that could help, assuming that’s the issue.
EDIT: Found a different dsi bin file and DSi mode seems to work properly, yay!