Is a firmware update required to read APFS volumes in Startup Manager in not so young Macs? Last week I plugged an SSD with a bootable APFS volume with macOS into the USB port of a Macbook Pro 2010.  It wasn’t seen in startup manager, as I had suspected.  This macbook had never been used as it had been stored in an exposition, so its firmware had never been updated.  It had its original and never used macOS 10.6 Snow Leopard installed.

Please correct me if this assumption is not correct:

Any mac whose firmware was never updated after APFS was first launched by Apple won’t see APFS volumes in startup manager because the logic board firmware wouldn’t be able to read APFS.  If any mac with installed macOS releases such as Yosemite, El Cap, Sierra can see a bootable APFS volume in startup manager, it’s because it’s firmware has been updated to be able to read and write APFS.   Is this correct??? 
The 2010 macbook mentioned above admits macOS up to the latest release (now High Sierra).

Any clarification much welcome.
When High Sierra is installed, any necessary firmware updates are applied. The has to be true, because any Mac built before APFS would not be able to boot from a APFS container partition without a firmware update.

However, even if a Mac as the firmware update which include APFS, this does not automatically mean that versions of macOS (OS X) prior to High Sierra would be able to access APFS volumes. Apple would have to provide an APFS software update for older versions of macOS (OS X). I do not believe Apple has done this.

This is also true for Windows users. Many Ask Different users have reported failures to read APFS volumes from Windows and the inability to boot back to macOS High Sierra directly from Windows.

As for your assumption. I can not provide a proper answer since your reference to a Startup Manager is ambiguous. Each Mac has a firmware Startup Manager, but I think you are referring to the  Startup Disk pane from the macOS (OS X) System Preferences application.