To backup your firmware type:

MTKFLASH "x" R /B /M filename.bin

Where "x" is a number from 1 to 4:

1: Primary master
2: Primary slave
3: Secondary master
4: Secondary slave

R is for Read out the firmware
/B is to save the firmware as a binary file
/M is to Merge the whole firmware into one file.
filename.bin is just an example, name your file so
it will be easy to find back. I use the firmware version
as the name like firmware version XS0U will be xs0u.bin.
Don't have the filename longer than 8 characters + bin extension.

The readout process will take a while depending on the flashrom size,
the flashrom is normally 128k, 256k or 512k. So just be patient.
Note that it's counting from 0% to 100% several times!