Is it possible to make a bootable USB stick for a Macbook Pro to do firmware update?
I can confirm that this process works:

https://fuerstnet.de/en/upgrade-crucial-m4-ssd-firmware-mac-without-cd

There are many steps to follow, both in creating the usb disk, then in booting from it, but it took me to the firmware updater as promised. After all that effort, my disk was already up-to-date, but it did work. 

Crucial support says there is no option for people without a CD drive, but there is. You just have to be willing to carefully follow some instructions and be sure you're not writing to the wrong disk.