Eric, I just checked the checkbox for Hide Master Password Field. However, I made two observations. On is that the MPW does get hidden in the Advanced Options panel. It does now get hidden in the Basic Options Panel, though.
I do not know if the second observation is a bug, or just a suggestion. When you close FF, after entering the MPW, it is no longer hidden. I think that this global setting should be persistent, if one does not choose to save the MPW to disk, as I do.