Stop threading by subject
From MozillaZine Knowledge Base
(This page was written for Thunderbird but also applies to SeaMonkey.)
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 set the preference mail.thread_without_re to false in the Thunderbird Config Editor (accessed by a button in the Advanced preferences) or the SeaMonkey about:config. Once the list of preferences is displayed, type thread in the Filter box to get more easily at this pref, and at the other two named below.
Threading by subject (and "Re: <subject>) is the default for various reasons, as stated in this 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. (In SeaMonkey, however, this preference defaults to false.)
You can also choose to thread only by header references by setting the preference mail.strict_threading to true in the same way.
There is also a third "mail threading" preference, namely mail.correct_threading.