On Mon, 2016-06-20 at 11:13 +0930, Stephen Davies wrote:
The first thing that happens when I request the reboot is that I get a message "GDBus.Error:org.freedesktop.DBus.Error.LimitsExceeded Maximum number of connections for UID 0 has been reached".
When the system does restart, I get a pop-up saying that Thunderbird is already running.
Perhaps, previously, you have saved the session on exit, and had Thunderbird running at that time.
If you can't find how to control start-up applications, you could try quitting all current apps, activate the save-session feature, and log out.