Avaliado com 5 de 5 estrelas

Thanks @Tinnyx, works beautifully.

Avaliado com 4 de 5 estrelas

Hey @Tinnyx, I've been using this plugin for years now and I'm not planning to stop doing so despite it no longer works with the current version of TB.
Fortunately it turns out it is enough the update the plugin's metadata as described here: https://developer.thunderbird.net/add-ons/updates/overlays

I took a stab at it and here is a manifest.json which works for me (please note that I've added some icon sizes):
{
"manifest_version": 2,
"applications": {
"gecko": {
"id": "[email protected]",
"strict_min_version": "68.0"
}
},
"name": "Keep in Taskbar",
"description": "A simple plugin that makes the Close button behave exactly like the Minimize one.",
"version": "2.0",
"author": "Martin Žilák",
"icons": {
"32": "chrome/content/icon32x32.png",
"48": "chrome/content/icon48x48.png"
},
"legacy": {
"type": "xul"
}
}
Would you care to release a new version, using the above manifest? (I hereby give up any rights to it, so please use it as you see fit.)
Thank you,
Michal Růžička

Esta análise é para uma versão anterior do extra (1.11). 

Thank you,
I have submitted the updated version you provided, it should be up and running in no time. I no longer use Thunderbird so I don't really keep track of the versions, but notifying me like this works and I'll try to keep the add-on updated.