Isolated data point: I have a couple of accounts, some of them have more than 50k messages. I noticed Thunderbird's auto-update wouldn't trigger so I manually installed the macOS .dmg with no issues. The previous Thunderbird profile was loaded as usual (with the folders on an external disk)

I have to say that the new look and feel is definitely better but it looked much nicer in the screenshots, I don't know why, I don't think it's a technical thing, perhaps my own expectations.