Thank you for the suggestions!
Opera browser so impresses me with its lightness, speed, and keyboard control, but I was leery of potentially bogging down performance with email-handling. But, importing Thunderbird account settings and mailbox was incredibly easy and quick! And the interface makes some things convenient. Still, I worry about slowing down Opera: I deeply rely on it as a featherweight-yet-featureful graphical web browser. So I look forward to trying Sylpheed Claws. I found claws-mail and many plugins for it in the testing repo. I hope it's stable enough.
Many, many thanks