Comment # 4 on bug 1189814 from Neil Rickert

Interesting.  I have never tested that.  I have just left that flag clear no
matter what.

I'm actually using a VM with ia32 architecture.  So I install there for legacy
booting, because the installer does not support ia32.  And then I had to setup
booting separately.  The installer wanted to set the pmbr_boot flag, but I said
to not do that.  Apparently, that was a wise choice.