I'm reasonably confident that firmware/boot and drivers are orthogonal; consider that there are PCs with normal UEFI boot that will happily boot any Linux image you want but it'll come up without drivers because nobody's written them, or on the other hand the Raspberry Pi has open source drivers[0] but the boot situation is still its own bizarro thing that's unlike any other machine so you can't boot the same USB stick on a Pi and anything else[1].

[0] Weirdly not all upstreamed last I looked, but mostly.

[1] I mean, you might be able to put a Pi-compatible bootloader and, say, a UEFI+ARM bootloader on the same stick, but I mean with a single boot path.