When Thunderbird status is set to "offline" at startup, user will not see the offline notification. By default, the status is "online" at startup (Thunderbird 52 and older).
To manually set "offline" at startup, do the following.
- In Thunderbird, go to Preferences from Edit in menu bar or hamburger menu (the right-most icon) in the toolbar.
- Go to Advanced and Network & Disk Space.
- Under Offline, click Offline... and "Offline Settings" window will appear.
- Under "Manual state when starting up", select
- Click OK to finish.
The offline notification will no longer appear at startup. Regardless of offline settings, when Thunderbird is already running on desktop, the offline notification does not seem to appear even when the offline is enabled or disabled from status bar or menu bar.
Previously, the offline notification seems to be inconsistent. Even when the status is "Automatically follow detected online state", user may still see offline notification (I experienced this).
With Thunderbird 60 and newer, no need to disable anything.
By new default, the status is "Automatically follow detected online state" at startup. Unlike older releases, the behaviour is now consistent; user will not see any offline notification at startup, unless user has clicked on the "Inbox" folder.
Tested with Mozilla Thunderbird 52.6.0 and 60.2.1 on Linux.