Stop threading by subject
From MozillaZine Knowledge Base
(This page was written for Thunderbird but also applies to SeaMonkey.)
Messages can be grouped into threads based on common Subjects, and/or by information stored in the References/In-Reply-To headers. There are many e-mail clients that do not add In-Reply-To and References headers to messages, which is the typical way to determine a thread. Threading based on the subject can be annoying when you get unrelated messages with common subjects such as "Hi!" or "Report!". By default Thunderbird tries to groups messages into threads in the following order:
There are several settings that manage threading that can be changed using the Config Editor at Tools -> Options -> Advanced -> General.
If you want to stop all threading by Subject set mail.strict_threading true.