Unfortunately, at least for me, I've had to mostly switch away from Thunderbird. It has becoming rather frustratingly slow. For example, if I archive an email, it takes 10 to 15 seconds for this to be reflected in the interface. When I have 50 or so emails to deal with, this adds up to a surprising amount of time, and becomes rather frustrating. (I suspect having large volumes of email is part of the issue.)
On the much needed feature side of things, other email clients have the very useful feature of showing the complete conversation history (across all accounts) in a sidebar. This alone has been a compelling reason to switch email clients, though, the real reason I switched was the extremely slow interface.
I sincerely hope these changes will make Thunderbird usable for me again.
I don't know how your archive is, mine is a few tens of thousands of messages, many of them with attachments, it works OK, fast and no problems at all.
After these news, I'm searching for an alternative right away. I won't touch a "rewriting from scratch" piece of software with a ten feet pole. Very disappointing.
Suggestions very welcome.
They know they have a huge user base, including enterprise users. They can do it right and modernize the UI without breaking your workflow. Maybe they will propose compact views and everything. They already have such options.
I've been using Thunderbird for 2005 and like it as is, but I wouldn't mind some fresh air. I'd also love being able to convince my younger relatives to adopt Thunderbird but that somewhat cannot happen in its current state.
Thunderbird is also not Firefox and I would expect them not handle UI/UX changes differently. Worst case, it will remain customizable. I'm not quite happy with the current Firefox UI, but luckily, someone built the Lepton theme [1] which is perfect for me. Thunderbird will still be based on Gecko for the UI, and I'm sure it'll remain at least as customizable as Firefox, even if it involves some hackery.
If Thunderbird works well for you, just wait. Maybe you'll like the changes after all?
As for the suggestions I could suggest KMail, it seems good, and would integrate perfectly with my KDE Plasma desktop environment, though I have been trapped in Thunderbird for more than a decade now.