Hi scrubbicus,
I saw your thread in the Software forum
HERE and thought I'd answer in this thread as it's a continuation of the topic.
That's not normal behavior. Using
recent:username should only download the messages once. Once T-bird has them, they should be listed in the popstate control file and ignored on future connections with the server.
By any chance are you changing it back and forth from
recent:username? That might cause the popstate file to reset like it's connecting for the first time to the server.