Tor Messenger is built from the same platform (XULRunner) as Tor Browser and, as such, they share a common codebase. However, they're two different applications.
Tor Messenger comes packaged with a copy of the tor daemon and launches it at startup. More advanced usage is available, same as in Tor Browser. Please see the documentation about "Using a system-installed Tor process with Tor Messenger",
More information about Tor Messenger can be found at,
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