Revise prefs adopted from TorBirdy
When we adopted the prefs from TorBirdy in #17220, a few things were left unclear, marked with XXX in config/chroot_local-includes/etc/thunderbird/pref/thunderbird.js.
- Affected tool set to Email Client
I've flagged this as FT work because cleaning this up would ease maintaining this set of prefs.
I propose we drop the target version though, as this is not a regression and IMO it's not particularly more urgent than many other FT tickets with no target version.