I looked around for a solution that still works in 16.04. I found that previously offered solutions including the one which was accepted were for older versions of Ubuntu.
The results were Thunderbird would open but not minimised.
There are alternative solutions here and here that explain ways to do this, they are general purpose scripts which can be adapted. However, it's a bit complex for the task and not tailored to Thunderbird.
I achieved the desired behaviour in the following way:
1) Install
alltray
which provides an easy method to open Thunderbird at startup:2) Add to Startup Applications:
This will open Thunderbird at startup but it will not be minimised.
----- UPDATE
Minimize on Start and Close
IS NO LONGER SUPPORTED -----3) Then installMinimize on Start and Close
in Thunderbird (Tools->Add-ons->Get Add-ons)This needs to be configured (Tools->Add-ons->Extensions->Preferences) as follows:--------------------------------------------------
3) Then install
Keep in Taskbar
and restart Thunderbird.This is a combination of previous suggestions from various locations that works in Ubuntu 16.04.
I thought I would share this to provide an easy, updated method for others.