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luca A very new fork of Thunderbird, so basically you're still using Thunderbird. Only, the makers claim it's better, because it gets updates that Thunderbird doesn't. Not sure what updates those are and why Thunderbird doesn't get those...
What I see is a project by, what they themselves call, a small team, complaining about the large number of open tickets in Thunderbird (which they forked and thus inherited). They don't help users, they only concentrate on fixing bugs they find important.
They can move fast by claiming to have fixed bugs X, Y and Z and pointing out that Thunderbird lags behind, while at the same time complaining how it took TB so long to fix "formal quality assurance", even though they themselves don't have that.
Let's see if it still exists in, say, a year and how their claims of being better than TB hold up against some scrutiny 😉