Calendar disappeared from the interface - I no longer can add/remove calendar, nor display it
I'm running TB Beta 42.0 and Calendar features just aren't there - I can't install Lightning indeed as it's no longer compatible ( because it's been integrated natively to TB, right ?)
Looking at the advanced settings, I can see my calendar related setting are still there, though I don't know how to make it going back.
In settings, the calendar section also vanished (that may happened a while ago, I can't tell since when it's no longer there - just noticed recently)
Opaite Mbohovái (1)
I can't install Lightning indeed as it's no longer compatible
You do need Lightning 4.4b2. https://addons.mozilla.org/en-US/thunderbird/addon/lightning/versions/beta
Uninstall any version which may still visible in Add-ons Manager first.
because it's been integrated natively to TB, right ?
No, integrated means Lightning ships with Thunderbird, but it's still an add-on.