Thunderbird mail client has implemented an indicator that notifies about new emails through an icon in the system tray. This implementation is based on the StatusNotifierItem protocol, originally developed by the KDE project (GNOME support is provided through an extension). A page to track work on this issue was created in November 1999. Unfortunately, the fix could not be completed in time for inclusion in the recent Thunderbird 128 ESR release. The code for the new feature is written in Rust.
Source: opennet.ru
