OK, closing out this thread and linking it to another. The root cause of the problem I reported is that on the MBT board, the ZFS module combined with the i915 HDMI module exhausts the available EFI staging area. I don't fully understand the details, but thanks to help from @stephenw10 the solution is relatively simple: just reinstall pfSense and use UFS for the file system rather than ZFS. I have confirmed this works with CE 2.5.1.
If you want to see additional details, this thread has more information.
So glad that this issue can finally be put to bed!