Thunderbird Tray Icon

Systray-x with the Thunderbird app is working good on Linux.

SysTray-X is a system tray extension for Thunderbird 68+. The addon uses the WebExtension API's to control an external system dependent system tray application.

The addon and system tray application can do:

custom new mail icon
display number of unread / new mails
show / hide Thunderbird (minimize)
minimizing hides to tray (remove Thunderbird from taskbar, pager and switcher when minimized)
minimize on startup
minimize on close

Ubuntu repository

Add the repository

20.10

wget -q https://download.opensuse.org/repositories/home:/Ximi1970:/Mozilla:/Add-ons/xUbuntu_20.10/Release.key
sudo mv -f  Release.key  /etc/apt/trusted.gpg.d/Systray-x.Ximi1970.asc
sudo bash -c 'echo "deb https://download.opensuse.org/repositories/home:/Ximi1970:/Mozilla:/Add-ons/xUbuntu_20.10 ./" > /etc/apt/sources.list.d/systray-x.list'
sudo apt update

20.04 LTS

wget -q https://download.opensuse.org/repositories/home:/Ximi1970:/Mozilla:/Add-ons/xUbuntu_20.04/Release.key
sudo apt-key add Release.key
sudo bash -c 'echo "deb https://download.opensuse.org/repositories/home:/Ximi1970:/Mozilla:/Add-ons/xUbuntu_20.04 ./" > /etc/apt/sources.list.d/systray-x.list'
sudo apt update

18.04 LTS

wget -q https://download.opensuse.org/repositories/home:/Ximi1970:/Mozilla:/Add-ons/xUbuntu_18.04/Release.key
sudo apt-key add Release.key
sudo bash -c 'echo "deb https://download.opensuse.org/repositories/home:/Ximi1970:/Mozilla:/Add-ons/xUbuntu_18.04 ./" > /etc/apt/sources.list.d/systray-x.list'
sudo apt update

16.04 LTS

wget -q http://download.opensuse.org/repositories/home:/Ximi1970:/Mozilla:/Add-ons/xUbuntu_16.04/Release.key
sudo apt-key add Release.key
sudo bash -c 'echo "deb http://download.opensuse.org/repositories/home:/Ximi1970:/Mozilla:/Add-ons/xUbuntu_16.04 ./" > /etc/apt/sources.list.d/systray-x.list'
sudo apt update

Install

Installing the SysTray-X addon and companion app package:

sudo apt install systray-x

For other distros see https://github.com/Ximi1970/systray-x#package

My custom icon

Source