- is there any Mac on which firmware can not be downgraded?
I see that for at least one type of MacBook Pro, firmware can be downgraded √
For the MacBookPro7,1:
- MacBook Pro EFI Firmware Update 2.0, boot ROM MBP71.0039.B0B
- MacBook Pro (13-inch, Mid 2010) EFI Firmware Update 2.5, boot ROM MB71.0039.B0E with support for OS X Recovery.
In some edge cases, the firmware that supports OS X Recovery prevents reinstallation of the the OS that was originally provided with the Mac. For a Mac that is to be sold, this is undesirable.
There may be other edge cases where a downgrade of firmware is either required or desirable.