When you replace CD/DVD with SSD and want to update firmware of yours SSD, but can neither burn ISO, downloaded from official website, nor boot from this CD..
Even you have created from ISO boot usb stick – Mac does not see when pressing Alt while booting – it needs UEFI boot loader.

Prepare USB Drive:
Connect yours  pendrive.
Open terminal window.
Determine which device it use in system. In Linux it will be sdX , in OSX – diskN

df -h 
/dev/sdX1 1,4G 1,4G 0 100% /media/user/Transcend8G

then unmount ( do not eject! ) usb drive

sudo umount /dev/sdX1

Download UEFI usb stick image with Crucial MX300 Firmware updater M0CR060

(UEFI usb stick image with Crucial Firmware updater M0CR050 from  16 May 2017 )

Write image to yours USB drive
Caution! Be careful!
1. You should write to drive (sdX/diskN), not to partition (sdX1,diskNs0)
2. If you specify wrong device – you WILL destroy your OS or data!

sudo dd if=uefi-mx300-M0CR060-usb-boot.raw of=/dev/sdX bs=8K

Now you can mount it back.

Check for latest Crucial SSD firmware at http://www.crucial.com/usa/en/support-ssd-firmware
If it have new version – download it , unpack, open downloaded fimware ISO and replace
/boot and /cde folders on USB drive.

Your UEFI USB Drive is ready to use.
Boot your mac with Alt key pressed and choose UEFIBOOT drive.
You will see two menu options:

* Micro Core Linux with waitusb=10 and superuser
* Micron Storage Director  FW update

Choose second one for FW update – and relax.