Having just set up the mozilla calendar plugin to launch when I open thunderbird, I was wondering how much memory I was going to be sacrificing to the GUI god. I was very pleasantly surprised to see that thunderbird.exe was consuming less than 2MB when minimized.

I put this down to MinimizeToTray. My guess is that it's calling SetProcessWorkingSetSize(GetCurrentProcess(), 0xffffffff, 0xffffffff) to swap the process out of memory when minimized.

Yep, after a while, the memory usage creeps back up again, as thunderbird does its background bits and pieces, like checking mail.