Don't override Debian's system-wide Thunderbird configuration

Due to config/chroot_local-includes/etc/thunderbird/pref/thunderbird.js we overwrite any pref changes made for the Debian package. I noticed this when working on #15942 (closed) and #15959 (closed) — the needed setting for intl.locale.requested was overwritten by us. While I’ve fixed that one there’s at least one more interesting setting:

// This setting is a workaround for some crashes inside the JS engine.
// By this Thunderbird will use more memory and acting slower as the sharing
// memory between interacting JS files is disabled.
pref ("javascript.options.baselinejit", false);

We should sync our config, and possibly consider using a patch instead so upstream changes are not silently lost like this.

Related issues

• Blocks #16209

Original created by @anonym on 16021 (Redmine)