Thunderbird is far from brilliant, it's adequate, which is high praise in a sea of absolute vile crap, but it still doesn't make it brilliant. Its search (a pretty fundamental feature of an email client at least to me) is absolutely god-awful if not completely broken, at least at the scale of mailboxes I have, and this doesn't seem like an uncommon problem that is limited to just me. I am forced to resort to grep or addons in order to do a proper search of any of my folders with any level of confidence that I am not missing those lovely disappearing messages, since random messages just aren't indexed in Thunderbird's search repeatably, and consistently over about a dozen years of using it, only about 50% of my messages get indexed to become searchable despite any amount of folder reorganization and rebuilding of indexes and deleting databases and other frustrating troubleshooting steps it never works properly.

I've lately been considering giving up on it entirely and moving to a TUI client for email. I might try Betterbird first to see if it's actually any better.