Thunderbird 159 not showing unread mail tray icon

Long time user and donator of Thunderbird.

Always keep program icon in tray and when a new/unread email arrives, a separate yellow icon would pop up next to it... Well, after updating from 158 to 159, that new/unread icon is no longer popping up. I'll click the main program icon to open Thunderbird to find that there's tons of new/unread mail sitting there, but that yellow tray icon never showed!

I am pretty adept at copying profiles from one PC to another (same version), but I've just spent the last 60 minutes trying to revert from 159 to 158. I keep getting profile errors when launching 158... I start by turning off auto updates in Settings, close the program. Then copy everything out of my 159 profile folder to the desktop. Run the installer for 158, close the program after install and first run. Then copy contents of my 159 profile into the current 158 profile folder. But when I run 158, I get an error like "you cannot run a newer profile on an older version, please create a new profile". Also tried deleting profiles.ini and removing all the profiles but that just yields a "no profile found" dead end.

Has anyone been able to roll back just one version in Thunderbird? What are the EXACT steps?

This post at PortableApps might help you to roll back to v158;

Hi TZR916,
The profile error you are seeing is an intentional block that Thunderbird uses to prevent older versions of the app from opening a profile modified by a newer version to protect against corruption.
You don't need to lose your data or start from scratch. You can force Thunderbird 158 to accept your updated profile using a native launch argument.

Part 1: How to successfully roll back to v158

1. Close Thunderbird completely.
2. Install Thunderbird 158 and make sure your version 159 profile data is copied into the active profile folder where you want it.
3. Press the Windows Key + R to open the Run dialog box.
4. Type or paste the following command and hit Enter:
   cmd
   thunderbird.exe --allow-downgrade
   
   Use code with caution.
   
   (Note: If Windows throws an error saying it can't find it, use the full path in quotes instead: "C:\Program Files\Mozilla Thunderbird\thunderbird.exe" --allow-downgrade)

This will bypass the version check, adapt your profile back down to 158, and open the app normally.

Part 2: Why the tray icon broke in v159
If you look at the development notes for version 159, the developers heavily modified the underlying system tray code to rewrite how unread mail indicators work (specifically to add tray icon badge support and message counts for Linux users) [r/Thunderbird]. It appears this code refactoring inadvertently caused a regression that broke or disabled the legacy yellow unread overlay icon functionality for Windows 11 users [r/Thunderbird].
Until Mozilla pushes out a hotfix to patch the tray regression, rolling back to 158 using the --allow-downgrade switch is your best bet to get the icon back!