How can I minimize Thunderbird 60.2.1 to tray after "MinimizeToTray revived" add-on stopped working?

Birdtray is a system tray new mail notification for Thunderbird 60+. Birdtray checks the unread email status directly by reading the Thunderbird email search database, which makes it immune to Thunderbird API changes. As a result, Birdtray is a great Firetray alternative that shouldn't break on Thunderbird updates.

You can find the Birdtray source and instructions for compiling from source on GitHub. There is also a PPA you can use to install it in Ubuntu 18.04, 18.10 or 19.04:

sudo add-apt-repository ppa:linuxuprising/apps
sudo apt-get update
sudo apt install birdtray

From / more info: Linux Uprising.

There is another add-on called "MinimizeToTray Reanimated" which claims to work with Thunderbird v60+.

MinimizeToTray Reanimated is an application mainly dedicated to Thunderbird to allow its minimization in the systray in one click. It is the update of MinimizeToTray Revived, which is not maintained anymore since the recent versions of Thunderbird.

Note: This add-on has not been reviewed by Mozilla yet.

Another alternative is FireTray. This is a fork from the original project that has been discontinued. The forked project supports Thunderbird 60+ and has other features such as supporting chat with a separate indicator icon etc.

Steps:

git clone https://github.com/firetray-updates/FireTray
cd FireTray/src
make build

Then load the firetray-x.y.z.xpi addon through Add-ons/Extensions -> Settings -> Install Add-on From File.

Tested on Ubuntu 16.04 and Thunderbird 60.2.1