Don't override Debian's system-wide Thunderbird configuration
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
We should sync our config, and possibly consider using a patch instead so upstream changes are not silently lost like this.