@eichkat3r @thunderbird You might want to take a look at https://github.com/Ximi1970/systray-x
I use it since Firetray was completely broken and am happy with it.
SysTray-X: A system tray extension for Thunderbird. Needs both the addon AND the companion app installed to work. Will not work with TB flatpaks or snaps. - Ximi1970/systray-x
, as well as EditEmailSubject MX especially for ScanToMail 
Moin :-) @thunderbird , i really loved to use Mnehny in the past, one playful feature i miss today is the text (en)coding ("ROT-13, Base64, UUEncode/UUDecode, encodeURI/decodeURI and similar, Unicode/number
representation, Kenny-Speak, 1337-Speak, Morse, Reverse,...") und its other useful features (Custom Headers for emails, X-Faces :D). Maybe it find some new developer
https://addons.thunderbird.net/en-US/thunderbird/addon/mnenhy/
- TbSync, plus
- Provider for CalDAV & CardDAV
so calendars and address books can be hosted on a @nextcloud server.