Enigmail commit d2a0c88d21a5e4f7ca76e7a1e9e50345e973da70 (part of 2.0~beta1 and
newer) renamed the pref and removed all usage of the old one.

On the Torbirdy front nothing happened yet to adjust to this change; let's
assume that they'll do s/protectHeaders/protectedHeaders/ too and turn the pref
into an integer (1: default: ask user at 1st time use / 0: off / 2: on) as well.
When we'll upgrade Torbirdy I'll check this (refs: #15661).