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#11
For the OP. You also have to go into Settings and Notifications to make sure that Thunderbird is allowed to be shown in the notification area, so that it is not off in the section of programs that you have to expand notification icons. Then just drag the icons where you want them to be in what order in the tray. I keep Outlook on my test machine as the last shown icon on the Left. Makes it easier to see when mail comes in, while I am using it.