The Lightning extension for Mozilla Thunderbird adds the long missing calendar component into the beloved mail and newsreader app.
It is, in a word, “rough.” However, it does work. It’s functional in the way that a two by four can be used as a writing utensil.
Unwieldy, it’s not very polished, you may get splinters, and there are just some things you can’t do with it—but it is functional as a calendar. I’m just letting you know, in case you wanted a calendar from the Mozilla family that integrates into Thunderbird.