Stop threading by subject
Thunderbird, by default, assumes all threads with the same subject line are in the same thread. This can be an annoyance for common subjects like "Hi!" or "Report" where the messages are unrelated.
To turn off threading by subject add the following to user.js:
# Stop threading messages together if they simply have the same subject.
user_pref("mail.thread_without_re", false);
user_pref("mail.thread_by_subject", false);
Use both, as comments in the bug report indicate the preference name may change.
Threading by subject (and "Re: <subject>) is the default for various reasons, as stated in the bug report. There are many e-mail clients that do not add "In-Reply-To" and "References" fields to messages, which is the typical way to determine a thread. When Thunderbird's default behavior was changed to not thread by subject, many users became unhappy, and threading by subject was reinstated as the default. It is doubtful this will ever be changed again.
