Bugzilla

FIX:
    - Right Click on Unified Inbox folder and select 'Properties'
    - Click on 'Choose'
    - check through the list and make sure the 'Inbox' is selected for each mail account.
    - click on 'OK'
    - select option 'Match all messages'
    - Click on 'Update' 

confirm this fixes the issue.

FIX:
    - Right Click on Unified Inbox folder and select 'Properties'
    - Click on 'Choose'
    - check through the list and make sure the 'Inbox' is selected for each mail account.
    - click on 'OK'
    - select option 'Match all messages'
    - Click on 'Update' 

confirm this fixes the issue.

User in forum reported - updated to 102.2.1, emails no longer show in Unified Inbox.
https://support.mozilla.org/en-US/questions/1387958
All emails show in the 'sub folder Inboxes'
Updating unchecked all checkboxes in the Unified folder Properties. 
This bug is not so much about the count of emails as to understanding why the 'Properties' had it's selected checkboxes deselected.
There was nothing to count.

It may also effect other created search result folders (not part of the 'Unified') - I'll report if come across any in Support Forum.