No such plugin?
If there isn't a plugin on Thunderbird's plugin page that addresses this, It may be because no such plugin yet exists.
Such a plugin might have to change the data stored for messages - for example to add a reference to a message header so that the "connection" between messages is remembered for next time you start Thunderbird and open that folder. There are indications† that Thunderbird might still use Mork for some data structures relating to threading. Mork has a reputation for being unreasonably obscure and difficult. The Mozilla folk have been struggling for decades to rid their codebase of Mork, they succeeded with Firefox. The use of Mork suggests that writing a plugin to manipulate the data would be challenging. Of course this is largely speculation on my part.
nsMsgThreads are a thin wrapper around their underlying mork
representation, keeping only keys/cached values in memory, with all
important/mutable data being stored in mork.
What can cause disconnected threads?
Outlook (or at least some versions thereof)
- Doesn't set the In-Reply-To or References headers (as it SHOULD)
- Can insert unexpected prefixes in the Subject line (e.g. "AW:" instead of "RE:")
This combination makes it near impossible to thread such messages. There are Thunderbird bug-reports that discuss how to deal with this (maybe this specific problem is resolved now?)
It is possible that other mail clients exist that do not produce these headers in a useful form.
What are these headers?
The contents of this field identify previous correspon-
dence which this message answers. Note that if message iden-
tifiers are used in this field, they must use the msg-id
The contents of this field identify other correspondence
which this message references. Note that if message identif-
iers are used, they must use the msg-id specification format.
3.6.4. Identification fields
Though optional, every message SHOULD have a "Message-ID:" field.
Furthermore, reply messages SHOULD have "In-Reply-To:" and
"References:" fields as appropriate, as described below.
"In-Reply-To:" field may be used to identify the message (or
messages) to which the new message is a reply, while the
"References:" field may be used to identify a "thread" of
What practical use is this answer?
Think of it as a placeholder until a better answer arrives.