Decide what to do with Memory Hole in Thunderbird

By installing Enigmail we get the Memory Hole feature, which is disabled by default (presumably since it’s still experimental and not widely supported). However, TorBirdy 2.3 (not in Debian as of writing this) enables this feature in Enigmail, so before we upgrade TorBirdy we have to decide whether we want to enable this.

Memory Hole degrades UX significantly if you are a recipient without support for it: all such emails will have the subject “Encrypted Message” and will lack threading (due to dropped “In-Reply-To”). So, in practice (given that very few clients support Memory Hole) this means that Tails users will be a PITA for the vast majority of non-Tails users.

I think we should proactively disable it by setting

user_pref("extensions.torbirdy.custom.extensions.enigmail.protectHeaders", false);

Torbirdy ticket about this: https://trac.torproject.org/projects/tor/ticket/28493

Related issues

• Related to #15201 (closed)
• Blocks #16209

Original created by @anonym on 13649 (Redmine)