Uhg. This is the exact opposite of what direction I want from Thunderbird. I want it to remain stable. I want it to remain looking like an email client I run on my native OS. I do not want giant white-space webshit design and an entire rebuild that makes all my extensions and well trained spam detection .dat unusable. Please stop changing things just to change. Thunderbird works.
Though I would not mind some refresh. Many (younger? and as young as me, actually) people who are used to webmails and mobile apps find it ugly, and I can see that.
The world is missing a fast and efficient desktop mail client that looks good and I'd be glad if Thunderbird were it.
There's a world in which the Thunderbird team understands they have a huge number of long time users and they should be careful to keep it usable for them, where they will take feedback and in which they build something that doesn't suck for people who don't now it yet, and this world could be ours.
Wait and see? I understand the concern and that many people are worried, I also don't quite like the trend of UIs with a lot of space lacking contrast everywhere, but I think a good outcome is actually likely.