I installed Thunderbird for the first time today, and removed it about an hour later. It sat there idle chewing up 67% of a CPU core.

That might have been Thunderbird doing its initial indexing of your existing messages. It's the first thing I disable after installing Thunderbird: go to Settings > General > Indexing (at the bottom of the page) and uncheck the setting for "Enable Global Search and Indexer." I always found it extremely slow and CPU intensive, and just not worth it, especially when you have IMAP server-side search.