So I'm trying to play some gameboy games (to be speedran too). It doesn't let me run games cos I need to download firmware (I think it was a BIOS one) but I'm struggling to find any. Does anyone know where I can find a download?

The only legal advice I can give you is rip the firmware from your console. Other than that, for speedrunning gambatte-speedrun or BGB are usually required by most games as they are usually the most accurate gameboy emulators.

Hello,

I would also like to encourage you away from Bizhawk for Game Boy speedruns. Both Gambatte and bgb have proven themselves to be more accurate emulators. (Here are some empirical testing results from emulators last year: https://www.sprintinglegs.com/emutiming/results.html )