I'm so glad for some news about Thunderbird's future. A conglomeration of little annoyances made me go hunting for an alternative client earlier in the year, and I just couldn't find one that would really replace it for me.
If any of the team are reading, I hope the search interface is on the radar for when the project is on stable footing again. That's Thunderbird's main area of weakness in my opinion.
I find recent emails with Thunderbird's search pretty reliably, but for indexing a large archive of emails I can definitely recommend Recoll. I have it run a scan daily as a cronjob and it finds absolutely everything.
Search would be fine (for my needs at least) if they'd just (add an option to) do one thing: Skip that stupid search tab and go straight to "open email as list". I don't want full email text sorted by "relevance" (whatever that means for a search on a couple of keywords). I want a list of email threads that match.
This is pretty much the only fly in my Thunderbird ointment. Otherwise it's been a great client for years and I look forward to using it for years more.
If any of the team are reading, I hope the search interface is on the radar for when the project is on stable footing again. That's Thunderbird's main area of weakness in my opinion.