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Select Inbox If Message List is empty: check to see if there are any Quick Filter Bar filters operating. Are any of the icons on the Quick Filter Bar highlighted? if yes, then click on the icon to remove the filter.

7/31/23: Upgraded from latest 102 to try the v115 Supernova. Discovered empty Inbox, Drafts, & Templates System: Windows 10 Home, version 22H2, build 19045.3208. 500Gb/1TB HDD free, 16Gb ram. Unified Inbox is a single pop3 account.

The Supernova (TB v115) downloaded and completed installation without any error notifications (as expected).

Sympton: After launching discovered that the Unified folder display for the Inbox, Drafts, & Templates were all empty. All local folder content displayed normally.

Switched to Troubleshoot Mode:

      Help > Troubleshoot Mode > Disable all add-ons + reset > continue to Troubleshooting Mode

All the missing messages were displayed! Exited Troubleshoot Mode. (Inbox empty again). Manually /individually disabled all three add-on extensions. Restarted Supernova. Inbox empty (argh!)

Found and attempted other mozilla help solutions without success:

> Unified folders > properties > ensured the problematic folder names were checked. (all three checked)
> Unified folders > properties > toggled OFF/ON the problematic folder names check. 

Found another article that mentioned deleting the content of the "smart mailboxes" folder C:\Users\<your user>\AppData\Roaming\Thunderbird\Profiles\<your profile>.default\Mail\smart mailboxes.

Closed Supernova, navigated to the folder above. Moved the folder content to an archive temp folder for safe keeping. This left the original "smart mailboxes" folder empty.

Launched Supernova and it rebuilt the smart mailboxes folder content.

Success! All missing messages are now displayed! Hope this helps