It really has. Thunderbird has become slower and slower and prone to lockups. The UI and feature set is still great but the performance is really horrific.
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.
