Linux' boot process is at its core designed around mechanisms that allow the bootloader to control binary execution: they're called the kernel command line (init=) and the initramfs. Granted, the bootloader is not the firmware, but since everyone is using GRUB these days it wouldn't be too hard for firmware to locate the right configuration pieces to overwrite. And since initramfs is by design unencrypted because you need it to decrypt the rest, it's trivial to get your evil.ko injected in there.
There's only so much you can do against evil firmware, unfortunately. Getting a coreboot/libreboot capable machine is the only real way out.
> It's just a shame that with Intel ME the Libreboot/Coreboot devs have given up on Intel machines. I'm very interested in the Purism Librem laptop but I have low hopes. Maybe the future will be ARM.
This isn't true. Libreboot is the fork that only works with completely free platforms, which the newer Intel chipsets make impossible. Coreboot is still very much committed to supporting new Intel chipsets, although the mainboard availability of course depends on what developers have time and interest for.
Google Chromebooks all ship with Coreboot, so they're fully supported and a great choice if you're trying to make the most free usable computer you can get. They can all run Linux, and some of them even Windows with a little more effort. They also have ARM-based ones, some of which are completely blob-free.
There's only so much you can do against evil firmware, unfortunately. Getting a coreboot/libreboot capable machine is the only real way out.