Might it be, in part, due to moving to newer versions of Firefox or Firefox-derived components under the hood? When Thunderbird was starting out, Firefox would have had something like a 10-15Mb memory footprint with no pages loaded and eaten approximately zero processor cycles while idle. It's, um, a lot bigger and hungrier now.

Not seeing, not at all, runs great, zero lockups, zero performance issues, looks great, how odd.